diff --git a/mod/forum/classes/local/vaults/post.php b/mod/forum/classes/local/vaults/post.php
index 186457a73b5..83264055303 100644
--- a/mod/forum/classes/local/vaults/post.php
+++ b/mod/forum/classes/local/vaults/post.php
@@ -437,19 +437,28 @@ class post extends db_table_vault {
             'where' => $privatewhere,
             'params' => $privateparams,
         ] = $this->get_private_reply_sql($user, $canseeprivatereplies, "mp");
+        [
+            'where' => $privatewhere2,
+            'params' => $privateparams2,
+        ] = $this->get_private_reply_sql($user, $canseeprivatereplies, "p");
 
         $sql = "
-            SELECT p.*
-            FROM {" . self::TABLE . "} p
+            SELECT posts.*
+            FROM {" . self::TABLE . "} posts
             JOIN (
-                SELECT mp.discussion, mp.created, MAX(mp.id) AS latestpostid
-                FROM {" . self::TABLE . "} mp
-                WHERE mp.discussion {$insql} {$privatewhere}
-                GROUP BY mp.discussion, mp.created
-                HAVING mp.created = MAX(mp.created)
-            ) lp on lp.discussion = p.discussion AND lp.latestpostid = p.id";
+                SELECT p.discussion, MAX(p.id) as latestpostid
+                FROM {" . self::TABLE . "} p
+                JOIN (
+                    SELECT mp.discussion, MAX(mp.created) AS created
+                    FROM {" . self::TABLE . "} mp
+                    WHERE mp.discussion {$insql} {$privatewhere}
+                    GROUP BY mp.discussion
+                ) lp ON lp.discussion = p.discussion AND lp.created = p.created
+                WHERE 1=1 {$privatewhere2}
+                GROUP BY p.discussion
+            ) plp on plp.discussion = posts.discussion AND plp.latestpostid = posts.id";
 
-        $records = $this->get_db()->get_records_sql($sql, array_merge($params, $privateparams));
+        $records = $this->get_db()->get_records_sql($sql, array_merge($params, $privateparams, $privateparams2));
         $entities = $this->transform_db_records_to_entities($records);
 
         return array_reduce($entities, function($carry, $entity) {
@@ -469,10 +478,10 @@ class post extends db_table_vault {
         $params = [];
         $privatewhere = '';
         if (!$canseeprivatereplies) {
-            $privatewhere = " AND ({$posttablealias}.privatereplyto = :privatereplyto OR " .
-                "{$posttablealias}.userid = :privatereplyfrom OR {$posttablealias}.privatereplyto = 0)";
-            $params['privatereplyto'] = $user->id;
-            $params['privatereplyfrom'] = $user->id;
+            $privatewhere = " AND ({$posttablealias}.privatereplyto = :{$posttablealias}_privatereplyto OR " .
+                "{$posttablealias}.userid = :{$posttablealias}_privatereplyfrom OR {$posttablealias}.privatereplyto = 0)";
+            $params[$posttablealias . '_privatereplyto'] = $user->id;
+            $params[$posttablealias . '_privatereplyfrom'] = $user->id;
         }
 
         return [
@@ -496,15 +505,19 @@ class post extends db_table_vault {
         list($insql, $params) = $this->get_db()->get_in_or_equal($discussionids, SQL_PARAMS_NAMED);
 
         $sql = "
-            SELECT p.*
-            FROM {" . self::TABLE . "} p
+            SELECT posts.*
+            FROM {" . self::TABLE . "} posts
             JOIN (
-                SELECT mp.discussion, mp.created, MIN(mp.id) AS firstpostid
-                FROM {" . self::TABLE . "} mp
-                WHERE mp.discussion {$insql}
-                GROUP BY mp.discussion, mp.created
-                HAVING mp.created = MIN(mp.created)
-            ) fp ON fp.discussion = p.discussion AND fp.firstpostid = p.id";
+                SELECT p.discussion, MIN(p.id) as firstpostid
+                FROM {" . self::TABLE . "} p
+                JOIN (
+                    SELECT mp.discussion, MIN(mp.created) AS created
+                    FROM {" . self::TABLE . "} mp
+                    WHERE mp.discussion {$insql}
+                    GROUP BY mp.discussion
+                ) lp ON lp.discussion = p.discussion AND lp.created = p.created
+                GROUP BY p.discussion
+            ) plp on plp.discussion = posts.discussion AND plp.firstpostid = posts.id";
 
         $records = $this->get_db()->get_records_sql($sql, $params);
         return $this->transform_db_records_to_entities($records);
diff --git a/mod/forum/tests/vaults_post_test.php b/mod/forum/tests/vaults_post_test.php
index 540715a3839..bd489047be6 100644
--- a/mod/forum/tests/vaults_post_test.php
+++ b/mod/forum/tests/vaults_post_test.php
@@ -793,15 +793,18 @@ final class vaults_post_test extends \advanced_testcase {
         $course = $datagenerator->create_course();
         [$teacher, $otherteacher] = $this->helper_create_users($course, 2, 'teacher');
         [$user, $user2] = $this->helper_create_users($course, 2, 'student');
+        $now = time();
         $forum = $datagenerator->create_module('forum', ['course' => $course->id]);
         [$discussion1, $post1] = $this->helper_post_to_forum($forum, $user);
-        $post2 = $this->helper_reply_to_post($post1, $user);
-        $post3 = $this->helper_reply_to_post($post1, $user);
-        $post4 = $this->helper_reply_to_post($post2, $user);
+        $post2 = $this->helper_reply_to_post($post1, $user, ['created' => $now + 100]);
+        $post3 = $this->helper_reply_to_post($post1, $user, ['created' => $now + 101]);
+        $post4 = $this->helper_reply_to_post($post2, $user, ['created' => $now + 102]);
         [$discussion2, $post5] = $this->helper_post_to_forum($forum, $user);
         $post6 = $this->helper_reply_to_post($post5, $user);
-        [$discussion3, $post7] = $this->helper_post_to_forum($forum, $user);
-        $post8 = $this->helper_post_to_discussion($forum, $discussion3, $teacher, [
+        $post7 = $this->helper_reply_to_post($post5, $user);
+        $post8 = $this->helper_reply_to_post($post6, $user);
+        [$discussion3, $post9] = $this->helper_post_to_forum($forum, $user);
+        $post10 = $this->helper_post_to_discussion($forum, $discussion3, $teacher, [
             'privatereplyto' => $user->id,
         ]);
 
@@ -813,22 +816,22 @@ final class vaults_post_test extends \advanced_testcase {
             [$discussion1->id, $discussion2->id], false);
         $this->assertCount(2, $ids);
         $this->assertEquals($post4->id, $ids[$discussion1->id]->get_id());
-        $this->assertEquals($post6->id, $ids[$discussion2->id]->get_id());
+        $this->assertEquals($post8->id, $ids[$discussion2->id]->get_id());
 
         $ids = $this->vault->get_latest_posts_for_discussion_ids($user,
             [$discussion1->id, $discussion2->id, $discussion3->id], false);
         $this->assertCount(3, $ids);
         $this->assertEquals($post4->id, $ids[$discussion1->id]->get_id());
-        $this->assertEquals($post6->id, $ids[$discussion2->id]->get_id());
-        $this->assertEquals($post8->id, $ids[$discussion3->id]->get_id());
+        $this->assertEquals($post8->id, $ids[$discussion2->id]->get_id());
+        $this->assertEquals($post10->id, $ids[$discussion3->id]->get_id());
 
         // Checks the user who doesn't have access to the private reply.
         $ids = $this->vault->get_latest_posts_for_discussion_ids($user2,
             [$discussion1->id, $discussion2->id, $discussion3->id], false);
         $this->assertCount(3, $ids);
         $this->assertEquals($post4->id, $ids[$discussion1->id]->get_id());
-        $this->assertEquals($post6->id, $ids[$discussion2->id]->get_id());
-        $this->assertEquals($post7->id, $ids[$discussion3->id]->get_id());
+        $this->assertEquals($post8->id, $ids[$discussion2->id]->get_id());
+        $this->assertEquals($post9->id, $ids[$discussion3->id]->get_id());
 
         // Checks the user with the private reply to.
         $ids = $this->vault->get_latest_posts_for_discussion_ids($user, [
@@ -839,8 +842,8 @@ final class vaults_post_test extends \advanced_testcase {
         ], false);
         $this->assertCount(3, $ids);
         $this->assertEquals($post4->id, $ids[$discussion1->id]->get_id());
-        $this->assertEquals($post6->id, $ids[$discussion2->id]->get_id());
-        $this->assertEquals($post8->id, $ids[$discussion3->id]->get_id());
+        $this->assertEquals($post8->id, $ids[$discussion2->id]->get_id());
+        $this->assertEquals($post10->id, $ids[$discussion3->id]->get_id());
     }
 
     /**
@@ -871,13 +874,16 @@ final class vaults_post_test extends \advanced_testcase {
         $user = $datagenerator->create_user();
         $course = $datagenerator->create_course();
         $forum = $datagenerator->create_module('forum', ['course' => $course->id]);
+        $now = time();
         [$discussion1, $post1] = $this->helper_post_to_forum($forum, $user);
-        $post2 = $this->helper_reply_to_post($post1, $user);
-        $post3 = $this->helper_reply_to_post($post1, $user);
-        $post4 = $this->helper_reply_to_post($post2, $user);
+        $post2 = $this->helper_reply_to_post($post1, $user, ['created' => $now + 100]);
+        $post3 = $this->helper_reply_to_post($post1, $user, ['created' => $now + 101]);
+        $post4 = $this->helper_reply_to_post($post2, $user, ['created' => $now + 102]);
         [$discussion2, $post5] = $this->helper_post_to_forum($forum, $user);
         $post6 = $this->helper_reply_to_post($post5, $user);
-        [$discussion3, $post7] = $this->helper_post_to_forum($forum, $user);
+        $post7 = $this->helper_reply_to_post($post5, $user);
+        $post8 = $this->helper_reply_to_post($post6, $user);
+        [$discussion3, $post9] = $this->helper_post_to_forum($forum, $user);
 
         $firstposts = $this->vault->get_first_post_for_discussion_ids([$discussion1->id]);
         $this->assertCount(1, $firstposts);
@@ -892,7 +898,7 @@ final class vaults_post_test extends \advanced_testcase {
         $this->assertCount(3, $firstposts);
         $this->assertEquals($post1->id, $firstposts[$post1->id]->get_id());
         $this->assertEquals($post5->id, $firstposts[$post5->id]->get_id());
-        $this->assertEquals($post7->id, $firstposts[$post7->id]->get_id());
+        $this->assertEquals($post9->id, $firstposts[$post9->id]->get_id());
 
         $firstposts = $this->vault->get_first_post_for_discussion_ids([
             $discussion1->id,
@@ -903,7 +909,7 @@ final class vaults_post_test extends \advanced_testcase {
         $this->assertCount(3, $firstposts);
         $this->assertEquals($post1->id, $firstposts[$post1->id]->get_id());
         $this->assertEquals($post5->id, $firstposts[$post5->id]->get_id());
-        $this->assertEquals($post7->id, $firstposts[$post7->id]->get_id());
+        $this->assertEquals($post9->id, $firstposts[$post9->id]->get_id());
     }
 
     /**