diff --git a/question/behaviour/behaviourbase.php b/question/behaviour/behaviourbase.php
index 3b9943f62bc..b321cf22d72 100644
--- a/question/behaviour/behaviourbase.php
+++ b/question/behaviour/behaviourbase.php
@@ -392,11 +392,20 @@ abstract class question_behaviour {
         $previouscomment = $this->qa->get_last_behaviour_var('comment');
         $newcomment = $pendingstep->get_behaviour_var('comment');
 
-        if (is_null($previouscomment) && !html_is_blank($newcomment) ||
-                $previouscomment != $newcomment) {
+        if ($previouscomment != $newcomment) {
+            // The comment has changed.
             return false;
         }
 
+        if (!html_is_blank($newcomment)) {
+            // Check comment format.
+            $previouscommentformat = $this->qa->get_last_behaviour_var('commentformat');
+            $newcommentformat = $pendingstep->get_behaviour_var('commentformat');
+            if ($previouscommentformat != $newcommentformat) {
+                return false;
+            }
+        }
+
         // So, now we know the comment is the same, so check the mark, if present.
         $previousfraction = $this->qa->get_fraction();
         $newmark = question_utils::clean_param_mark($pendingstep->get_behaviour_var('mark'));
diff --git a/question/behaviour/manualgraded/tests/walkthrough_test.php b/question/behaviour/manualgraded/tests/walkthrough_test.php
index c47e2053e1b..c7e1d0f6b10 100644
--- a/question/behaviour/manualgraded/tests/walkthrough_test.php
+++ b/question/behaviour/manualgraded/tests/walkthrough_test.php
@@ -432,6 +432,35 @@ class qbehaviour_manualgraded_walkthrough_testcase extends qbehaviour_walkthroug
         $this->check_current_mark(0);
     }
 
+    public function test_manual_graded_change_comment_format() {
+        global $PAGE;
+
+        // The current text editor depends on the users profile setting - so it needs a valid user.
+        $this->setAdminUser();
+        // Required to init a text editor.
+        $PAGE->set_url('/');
+
+        // Create an essay question.
+        $essay = test_question_maker::make_an_essay_question();
+        $this->start_attempt_at_question($essay, 'deferredfeedback', 10);
+
+        // Simulate some data submitted by the student.
+        $this->process_submission(array('answer' => 'This is my wonderful essay!', 'answerformat' => FORMAT_HTML));
+
+        // Finish the attempt.
+        $this->quba->finish_all_questions();
+
+        // Process an example comment and a grade of 0.
+        $this->manual_grade('example', 0, FORMAT_HTML);
+        // Verify the format is FORMAT_HTML.
+        $this->check_comment('example', FORMAT_HTML);
+
+        // Process the same grade and comment with different format.
+        $this->manual_grade('example', 0, FORMAT_MARKDOWN);
+        // Verify the format is FORMAT_MARKDOWN.
+        $this->check_comment('example', FORMAT_MARKDOWN);
+    }
+
     public function test_manual_graded_respects_display_options() {
         // This test is for MDL-43874. Manual comments were not respecting the
         // Display options for feedback.
diff --git a/question/engine/tests/helpers.php b/question/engine/tests/helpers.php
index 93d3ecb9a52..eec70bf123e 100644
--- a/question/engine/tests/helpers.php
+++ b/question/engine/tests/helpers.php
@@ -778,6 +778,20 @@ abstract class qbehaviour_walkthrough_test_base extends question_testcase {
         $this->quba = null;
     }
 
+    /**
+     * Asserts if the manual comment for the question is equal to the provided arguments.
+     * @param $comment Comment text
+     * @param $commentformat Comment format
+     */
+    protected function check_comment($comment, $commentformat) {
+        $actualcomment = $this->quba->get_question_attempt($this->slot)->get_manual_comment();
+
+        $this->assertEquals(
+                [$comment, $commentformat],
+                [$actualcomment[0], $actualcomment[1]]
+        );
+    }
+
     protected function check_current_state($state) {
         $this->assertEquals($state, $this->quba->get_question_state($this->slot),
             'Questions is in the wrong state.');