diff --git a/lib/questionlib.php b/lib/questionlib.php
index 8cb54e258b6..e578d7adbf1 100644
--- a/lib/questionlib.php
+++ b/lib/questionlib.php
@@ -354,8 +354,14 @@ function question_delete_question($questionid): void {
              WHERE q.id = ?';
     $questiondata = $DB->get_record_sql($sql, [$question->id]);
 
+    $questionstocheck = [$question->id];
+
+    if ($question->parent !== 0) {
+        $questionstocheck[] = $question->parent;
+    }
+
     // Do not delete a question if it is used by an activity module
-    if (questions_in_use([$question->id])) {
+    if (questions_in_use($questionstocheck)) {
         return;
     }
 
diff --git a/question/engine/tests/helpers.php b/question/engine/tests/helpers.php
index 59a8b50f846..b3865a4dec8 100644
--- a/question/engine/tests/helpers.php
+++ b/question/engine/tests/helpers.php
@@ -1301,6 +1301,24 @@ abstract class qbehaviour_walkthrough_test_base extends question_testcase {
         // Does not currently verify hint text.
         return new question_contains_tag_with_attribute('div', 'class', 'hint');
     }
+
+    /**
+     * Returns an expectation that a string contains a corrupted question notification.
+     *
+     * @return question_pattern_expectation an expectation for use with check_current_output.
+     */
+    protected function get_contains_corruption_notification() {
+        return new question_pattern_expectation('/' . preg_quote(get_string('corruptedquestion', 'qtype_multianswer'), '/') . '/');
+    }
+
+    /**
+     * Returns an expectation that a string contains a corrupted subquestion message.
+     *
+     * @return question_pattern_expectation an expectation for use with check_current_output.
+     */
+    protected function get_contains_corrupted_subquestion_message() {
+        return new question_pattern_expectation('/' . preg_quote(get_string('missingsubquestion', 'qtype_multianswer'), '/') . '/');
+    }
 }
 
 /**
diff --git a/question/type/multianswer/backup/moodle2/restore_qtype_multianswer_plugin.class.php b/question/type/multianswer/backup/moodle2/restore_qtype_multianswer_plugin.class.php
index 80865b11523..f51ec0272f8 100644
--- a/question/type/multianswer/backup/moodle2/restore_qtype_multianswer_plugin.class.php
+++ b/question/type/multianswer/backup/moodle2/restore_qtype_multianswer_plugin.class.php
@@ -110,7 +110,7 @@ class restore_qtype_multianswer_plugin extends restore_qtype_plugin {
             foreach ($sequencearr as $key => $question) {
                 $sequencearr[$key] = $this->get_mappingid('question', $question);
             }
-            $sequence = implode(',', $sequencearr);
+            $sequence = implode(',', array_filter($sequencearr));
             $DB->set_field('question_multianswer', 'sequence', $sequence,
                     array('id' => $rec->id));
             if (!empty($sequence)) {
diff --git a/question/type/multianswer/edit_multianswer_form.php b/question/type/multianswer/edit_multianswer_form.php
index 58a8f44349d..02a625cea4d 100644
--- a/question/type/multianswer/edit_multianswer_form.php
+++ b/question/type/multianswer/edit_multianswer_form.php
@@ -150,7 +150,8 @@ class qtype_multianswer_edit_form extends question_edit_form {
                 $storemess = '';
                 if (isset($this->savedquestiondisplay->options->questions[$sub]->qtype) &&
                         $this->savedquestiondisplay->options->questions[$sub]->qtype !=
-                                $this->questiondisplay->options->questions[$sub]->qtype) {
+                                $this->questiondisplay->options->questions[$sub]->qtype &&
+                        $this->savedquestiondisplay->options->questions[$sub]->qtype != 'subquestion_replacement') {
                     $this->qtypechange = true;
                     $storemess = ' ' . html_writer::tag('span', get_string(
                             'storedqtype', 'qtype_multianswer', question_bank::get_qtype_name(
@@ -276,6 +277,8 @@ class qtype_multianswer_edit_form extends question_edit_form {
                             case 'numerical':
                                 $parsableanswerdef .= 'NUMERICAL:';
                                 break;
+                            case 'subquestion_replacement':
+                                continue 2;
                             default:
                                 print_error('unknownquestiontype', 'question', '',
                                         $wrapped->qtype);
diff --git a/question/type/multianswer/lang/en/qtype_multianswer.php b/question/type/multianswer/lang/en/qtype_multianswer.php
index b51660ff1f3..ba4f575d18c 100644
--- a/question/type/multianswer/lang/en/qtype_multianswer.php
+++ b/question/type/multianswer/lang/en/qtype_multianswer.php
@@ -27,6 +27,7 @@ $string['confirmquestionsaveasedited'] = 'I confirm that I want the question to
 $string['confirmsave'] = 'Confirm then save {$a}';
 $string['correctanswer'] = 'Correct answer';
 $string['correctanswerandfeedback'] = 'Correct answer and feedback';
+$string['corruptedquestion'] = 'This question is corrupted and contains subquestions that are not present in your system.';
 $string['decodeverifyquestiontext'] = 'Decode and verify the question text';
 $string['invalidmultianswerquestion'] = 'Invalid embedded answers (Cloze) question ({$a}).';
 $string['layout'] = 'Layout';
@@ -36,6 +37,7 @@ $string['layoutmultiple_vertical'] = 'Vertical column of checkboxes';
 $string['layoutselectinline'] = 'Drop-down menu in-line in the text';
 $string['layoutundefined'] = 'Undefined layout';
 $string['layoutvertical'] = 'Vertical column of radio buttons';
+$string['missingsubquestion'] = 'This subquestion is missing from your system and cannot be displayed.';
 $string['nooptionsforsubquestion'] = 'Unable to get options for question part # {$a->sub} (question->id={$a->id})';
 $string['noquestions'] = 'The Cloze(multianswer) question "<strong>{$a}</strong>" does not contain any question';
 $string['pleaseananswerallparts'] = 'Please answer all parts of the question.';
diff --git a/question/type/multianswer/question.php b/question/type/multianswer/question.php
index de5522455a6..2910fca66d9 100644
--- a/question/type/multianswer/question.php
+++ b/question/type/multianswer/question.php
@@ -110,7 +110,7 @@ class qtype_multianswer_question extends question_graded_automatically_with_coun
             $fractionmax += $subq->defaultmark;
             $fractionsum += $subq->defaultmark * $subq->get_min_fraction();
         }
-        return $fractionsum / $fractionmax;
+        return $fractionsum / (!empty($this->subquestions) ? $fractionmax : 1);
     }
 
     public function get_max_fraction() {
@@ -120,7 +120,7 @@ class qtype_multianswer_question extends question_graded_automatically_with_coun
             $fractionmax += $subq->defaultmark;
             $fractionsum += $subq->defaultmark * $subq->get_max_fraction();
         }
-        return $fractionsum / $fractionmax;
+        return $fractionsum / (!empty($this->subquestions) ? $fractionmax : 1);
     }
 
     public function get_expected_data() {
diff --git a/question/type/multianswer/questiontype.php b/question/type/multianswer/questiontype.php
index 07462f07c24..8e511ed474b 100644
--- a/question/type/multianswer/questiontype.php
+++ b/question/type/multianswer/questiontype.php
@@ -38,18 +38,86 @@ require_once($CFG->dirroot . '/question/type/numerical/questiontype.php');
  */
 class qtype_multianswer extends question_type {
 
+    /**
+     * Generate a subquestion replacement question class.
+     *
+     * Due to a bug, subquestions can be lost (see MDL-54724). This class exists to take
+     * the place of those lost questions so that the system can keep working and inform
+     * the user of the corrupted data.
+     *
+     * @return question_automatically_gradable The replacement question class.
+     */
+    public static function deleted_subquestion_replacement(): question_automatically_gradable {
+        return new class implements question_automatically_gradable {
+            public $qtype;
+
+            public function __construct() {
+                $this->qtype = new class() {
+                    public function name() {
+                        return 'subquestion_replacement';
+                    }
+                };
+            }
+
+            public function is_gradable_response(array $response) {
+                return false;
+            }
+
+            public function is_complete_response(array $response) {
+                return false;
+            }
+
+            public function is_same_response(array $prevresponse, array $newresponse) {
+                return false;
+            }
+
+            public function summarise_response(array $response) {
+                return '';
+            }
+
+            public function un_summarise_response(string $summary) {
+                return [];
+            }
+
+            public function classify_response(array $response) {
+                return [];
+            }
+
+            public function get_validation_error(array $response) {
+                return '';
+            }
+
+            public function grade_response(array $response) {
+                return [];
+            }
+
+            public function get_hint($hintnumber, question_attempt $qa) {
+                return;
+            }
+
+            public function get_right_answer_summary() {
+                return null;
+            }
+        };
+    }
+
     public function can_analyse_responses() {
         return false;
     }
 
     public function get_question_options($question) {
-        global $DB, $OUTPUT;
+        global $DB;
 
         parent::get_question_options($question);
         // Get relevant data indexed by positionkey from the multianswers table.
         $sequence = $DB->get_field('question_multianswer', 'sequence',
                 array('question' => $question->id), MUST_EXIST);
 
+        if (empty($sequence)) {
+            $question->options->questions = [];
+            return true;
+        }
+
         $wrappedquestions = $DB->get_records_list('question', 'id',
                 explode(',', $sequence), 'id ASC');
 
@@ -59,13 +127,19 @@ class qtype_multianswer extends question_type {
             $val++;
         });
 
-        // If a question is lost, the corresponding index is null
-        // so this null convention is used to test $question->options->questions
-        // before using the values.
-        // First all possible questions from sequence are nulled
-        // then filled with the data if available in  $wrappedquestions.
+        // Due to a bug, questions can be lost (see MDL-54724). So we first fill the question
+        // options with this dummy "replacement" type. These are overridden in the loop below
+        // leaving behind only those questions which no longer exist. The renderer then looks
+        // for this deleted type to display information to the user about the corrupted question
+        // data.
         foreach ($sequence as $seq) {
-            $question->options->questions[$seq] = '';
+            $question->options->questions[$seq] = (object)[
+                'qtype' => 'subquestion_replacement',
+                'defaultmark' => 1,
+                'options' => (object)[
+                    'answers' => []
+                ]
+            ];
         }
 
         foreach ($wrappedquestions as $wrapped) {
@@ -216,6 +290,10 @@ class qtype_multianswer extends question_type {
             $i += 1;
         }
         foreach ($questiondata->options->questions as $key => $subqdata) {
+            if ($subqdata->qtype == 'subquestion_replacement') {
+                continue;
+            }
+
             $subqdata->contextid = $questiondata->contextid;
             if ($subqdata->qtype == 'multichoice') {
                 $answerregs = array();
diff --git a/question/type/multianswer/renderer.php b/question/type/multianswer/renderer.php
index 5993da13f02..27c31e9a6fd 100644
--- a/question/type/multianswer/renderer.php
+++ b/question/type/multianswer/renderer.php
@@ -46,17 +46,38 @@ class qtype_multianswer_renderer extends qtype_renderer {
 
         $output = '';
         $subquestions = array();
+
+        $missingsubquestions = false;
         foreach ($question->textfragments as $i => $fragment) {
             if ($i > 0) {
                 $index = $question->places[$i];
+                $questionisvalid = !empty($question->subquestions[$index]) &&
+                                 $question->subquestions[$index]->qtype->name() !== 'subquestion_replacement';
+
+                if (!$questionisvalid) {
+                    $missingsubquestions = true;
+                    $questionreplacement = qtype_multianswer::deleted_subquestion_replacement();
+
+                    // It is possible that the subquestion index does not exist. When corrupted quizzes (see MDL-54724) are
+                    // restored, the sequence column of mdl_quiz_multianswer can be empty, in this case
+                    // qtype_multianswer::get_question_options cannot fill in deleted questions, so we need to do it here.
+                    $question->subquestions[$index] = $question->subquestions[$index] ?? $questionreplacement;
+                }
+
                 $token = 'qtypemultianswer' . $i . 'marker';
                 $token = '<span class="nolink">' . $token . '</span>';
                 $output .= $token;
                 $subquestions[$token] = $this->subquestion($qa, $options, $index,
                         $question->subquestions[$index]);
             }
+
             $output .= $fragment;
         }
+
+        if ($missingsubquestions) {
+            $output = $this->notification(get_string('corruptedquestion', 'qtype_multianswer'), 'error') . $output;
+        }
+
         $output = $question->format_text($output, $question->questiontextformat,
                 $qa, 'question', 'questiontext', $question->id);
         $output = str_replace(array_keys($subquestions), array_values($subquestions), $output);
@@ -78,7 +99,7 @@ class qtype_multianswer_renderer extends qtype_renderer {
     }
 
     public function subquestion(question_attempt $qa,
-            question_display_options $options, $index, question_graded_automatically $subq) {
+            question_display_options $options, $index, question_automatically_gradable $subq) {
 
         $subtype = $subq->qtype->name();
         if ($subtype == 'numerical' || $subtype == 'shortanswer') {
@@ -99,6 +120,11 @@ class qtype_multianswer_renderer extends qtype_renderer {
                     $subrenderer = 'multichoice_vertical';
                 }
             }
+        } else if ($subtype == 'subquestion_replacement') {
+            return html_writer::div(
+                get_string('missingsubquestion', 'qtype_multianswer'),
+                'notifyproblem'
+            );
         } else {
             throw new coding_exception('Unexpected subquestion type.', $subq);
         }
diff --git a/question/type/multianswer/tests/question_type_test.php b/question/type/multianswer/tests/question_type_test.php
index a25b5ccce2b..d36a13e85a4 100644
--- a/question/type/multianswer/tests/question_type_test.php
+++ b/question/type/multianswer/tests/question_type_test.php
@@ -20,6 +20,7 @@ use qtype_multianswer;
 use qtype_multianswer_edit_form;
 use qtype_multichoice_base;
 use question_bank;
+use test_question_maker;
 
 defined('MOODLE_INTERNAL') || die();
 
@@ -360,4 +361,43 @@ class question_type_test extends \advanced_testcase {
             }
         }
     }
+
+    /**
+     * Test get_question_options.
+     *
+     * @covers \qtype_multianswer::get_question_options
+     */
+    public function test_get_question_options() {
+        global $DB;
+
+        $this->resetAfterTest(true);
+
+        $syscontext = \context_system::instance();
+        $generator = $this->getDataGenerator()->get_plugin_generator('core_question');
+        $category = $generator->create_question_category(['contextid' => $syscontext->id]);
+
+        $fromform = test_question_maker::get_question_form_data('multianswer', 'twosubq');
+        $fromform->category = $category->id . ',' . $syscontext->id;
+
+        $question = new \stdClass();
+        $question->category = $category->id;
+        $question->qtype = 'multianswer';
+        $question->createdby = 0;
+
+        $question = $this->qtype->save_question($question, $fromform);
+        $questiondata = question_bank::load_question_data($question->id);
+
+        $questiontodeletekey = array_keys($questiondata->options->questions)[0];
+        $questiontodelete = $questiondata->options->questions[$questiontodeletekey];
+
+        $this->assertCount(2, $questiondata->options->questions);
+        $this->assertEquals('shortanswer', $questiondata->options->questions[$questiontodeletekey]->qtype);
+
+        // Forcibly delete a subquestion to ensure get_question_options replaces it.
+        $DB->delete_records('question', ['id' => $questiontodelete->id]);
+        $this->qtype->get_question_options($questiondata);
+
+        $this->assertCount(2, $questiondata->options->questions);
+        $this->assertEquals('subquestion_replacement', $questiondata->options->questions[$questiontodeletekey]->qtype);
+    }
 }
diff --git a/question/type/multianswer/tests/walkthrough_test.php b/question/type/multianswer/tests/walkthrough_test.php
index 02ba5df4068..aaf525f7f18 100644
--- a/question/type/multianswer/tests/walkthrough_test.php
+++ b/question/type/multianswer/tests/walkthrough_test.php
@@ -16,14 +16,18 @@
 
 namespace qtype_multianswer;
 
+use qtype_multianswer;
+use question_bank;
 use question_display_options;
 use question_hint_with_parts;
 use question_state;
+use test_question_maker;
 
 defined('MOODLE_INTERNAL') || die();
 
 global $CFG;
 require_once($CFG->dirroot . '/question/engine/tests/helpers.php');
+require_once($CFG->dirroot . '/question/type/multianswer/questiontype.php');
 
 
 /**
@@ -530,4 +534,43 @@ class walkthrough_test extends \qbehaviour_walkthrough_test_base {
             $this->get_contains_partcorrect_expectation(),
             $this->get_does_not_contain_validation_error_expectation());
     }
+
+    /**
+     * Test corrupted question display.
+     *
+     * @covers \qtype_multianswer_renderer::subquestion
+     */
+    public function test_corrupted_question() {
+        global $DB;
+
+        $syscontext = \context_system::instance();
+        $generator = $this->getDataGenerator()->get_plugin_generator('core_question');
+        $category = $generator->create_question_category(['contextid' => $syscontext->id]);
+
+        $fromform = test_question_maker::get_question_form_data('multianswer', 'twosubq');
+        $fromform->category = $category->id . ',' . $syscontext->id;
+
+        $question = new \stdClass();
+        $question->category = $category->id;
+        $question->qtype = 'multianswer';
+        $question->createdby = 0;
+
+        $question = (new qtype_multianswer)->save_question($question, $fromform);
+        $questiondata = question_bank::load_question_data($question->id);
+        $questiontodeletekey = array_keys($questiondata->options->questions)[0];
+        $questiontodelete = $questiondata->options->questions[$questiontodeletekey];
+        $DB->delete_records('question', ['id' => $questiontodelete->id]);
+
+        question_bank::notify_question_edited($question->id);
+        $question = question_bank::load_question($question->id);
+
+        $this->start_attempt_at_question($question, 'deferredfeedback', 2);
+        $this->check_current_output(
+            $this->get_contains_marked_out_of_summary(),
+            $this->get_does_not_contain_feedback_expectation(),
+            $this->get_does_not_contain_validation_error_expectation(),
+            $this->get_contains_corruption_notification(),
+            $this->get_contains_corrupted_subquestion_message(),
+        );
+    }
 }
diff --git a/question/type/multianswer/version.php b/question/type/multianswer/version.php
index 9cdfc75d372..e15bd438a9e 100644
--- a/question/type/multianswer/version.php
+++ b/question/type/multianswer/version.php
@@ -26,7 +26,7 @@
 defined('MOODLE_INTERNAL') || die();
 
 $plugin->component = 'qtype_multianswer';
-$plugin->version   = 2021052500;
+$plugin->version   = 2021052500.01;
 
 $plugin->requires  = 2021052500;
 $plugin->dependencies = array(