diff --git a/lib/Chat/Parser/UserMention.php b/lib/Chat/Parser/UserMention.php
index 38a20a2de01..4982a1b8e97 100644
--- a/lib/Chat/Parser/UserMention.php
+++ b/lib/Chat/Parser/UserMention.php
@@ -78,6 +78,11 @@ public function parseMessage(Message $chatMessage): void {
 		$mentionTypeCount = [];
 
 		$mentions = $comment->getMentions();
+		// TODO This can be removed once getMentions() returns sorted results (Nextcloud 21+)
+		usort($mentions, static function ($m1, $m2) {
+			return mb_strlen($m2['id']) <=> mb_strlen($m1['id']);
+		});
+
 		foreach ($mentions as $mention) {
 			if ($mention['type'] === 'user' && $mention['id'] === 'all') {
 				$mention['type'] = 'call';
@@ -101,12 +106,10 @@ public function parseMessage(Message $chatMessage): void {
 			// index of the mentions of that type.
 			$mentionParameterId = 'mention-' . $mention['type'] . $mentionTypeCount[$mention['type']];
 
-			if (strpos($mention['id'], ' ') !== false || strpos($mention['id'], 'guest/') === 0) {
-				$placeholder = '@"' . $mention['id'] . '"';
-			} else {
-				$placeholder = '@' .  $mention['id'];
+			$message = str_replace('@"' . $mention['id'] . '"', '{' . $mentionParameterId . '}', $message);
+			if (strpos($mention['id'], ' ') === false && strpos($mention['id'], 'guest/') !== 0) {
+				$message = str_replace('@' . $mention['id'], '{' . $mentionParameterId . '}', $message);
 			}
-			$message = str_replace($placeholder, '{' . $mentionParameterId . '}', $message);
 
 			if ($mention['type'] === 'call') {
 				$messageParameters[$mentionParameterId] = [
diff --git a/tests/integration/features/chat/rich-messages.feature b/tests/integration/features/chat/rich-messages.feature
index 13b51323a8a..e4d07c26d2a 100644
--- a/tests/integration/features/chat/rich-messages.feature
+++ b/tests/integration/features/chat/rich-messages.feature
@@ -3,6 +3,7 @@ Feature: chat/public
     Given user "participant1" exists
     Given user "participant2" exists
     Given user "participant3" exists
+    Given user "participant3a" exists
 
   Scenario: message without enrichable references has empty parameters
     Given user "participant1" creates room "public room"
@@ -48,3 +49,14 @@ Feature: chat/public
     Then user "participant1" sees the following messages in room "public room" with 200
       | room        | actorType | actorId      | actorDisplayName         | message                                                                                | messageParameters |
       | public room | users     | participant1 | participant1-displayname | Mention to {mention-user1}, @unknownUser, {mention-user1} again and {mention-user2} | {"mention-user1":{"type":"user","id":"participant2","name":"participant2-displayname"},"mention-user2":{"type":"user","id":"participant3","name":"participant3-displayname"}} |
+
+  Scenario: message with mentions of subname users (uid1 is fully part of uid2)
+    Given user "participant1" creates room "public room"
+      | roomType | 3 |
+      | roomName | room |
+    When user "participant1" sends message "Mention to @participant3 and @participant3a" to room "public room" with 201
+    When user "participant1" sends message "Mention to @participant3a and @participant3" to room "public room" with 201
+    Then user "participant1" sees the following messages in room "public room" with 200
+      | room        | actorType | actorId      | actorDisplayName         | message                                                                                | messageParameters |
+      | public room | users     | participant1 | participant1-displayname | Mention to {mention-user1} and {mention-user2} | {"mention-user1":{"type":"user","id":"participant3a","name":"participant3a-displayname"},"mention-user2":{"type":"user","id":"participant3","name":"participant3-displayname"}} |
+      | public room | users     | participant1 | participant1-displayname | Mention to {mention-user2} and {mention-user1} | {"mention-user1":{"type":"user","id":"participant3a","name":"participant3a-displayname"},"mention-user2":{"type":"user","id":"participant3","name":"participant3-displayname"}} |
diff --git a/tests/php/Chat/Parser/UserMentionTest.php b/tests/php/Chat/Parser/UserMentionTest.php
index a25c136414e..afe9a2486d5 100644
--- a/tests/php/Chat/Parser/UserMentionTest.php
+++ b/tests/php/Chat/Parser/UserMentionTest.php
@@ -174,6 +174,80 @@ public function testGetRichMessageWithDuplicatedMention() {
 		$this->assertEquals($expectedMessageParameters, $chatMessage->getMessageParameters());
 	}
 
+	public function dataGetRichMessageWithMentionsFullyIncludedInOtherMentions() {
+		// Based on valid characters from server/lib/private/User/Manager.php
+		return [
+			['testUser', 'testUser1', false],
+			['testUser', 'testUser1', true],
+			['testUser', 'testUser_1', false],
+			['testUser', 'testUser_1', true],
+			['testUser', 'testUser.1', false],
+			['testUser', 'testUser.1', true],
+			['testUser', 'testUser@1', false],
+			['testUser', 'testUser@1', true],
+			['testUser', 'testUser-1', false],
+			['testUser', 'testUser-1', true],
+			['testUser', 'testUser\'1', false],
+			['testUser', 'testUser\'1', true],
+		];
+	}
+
+	/**
+	 * @dataProvider dataGetRichMessageWithMentionsFullyIncludedInOtherMentions
+	 */
+	public function testGetRichMessageWithMentionsFullyIncludedInOtherMentions(string $baseId, string $longerId, bool $quoted) {
+		$mentions = [
+			['type' => 'user', 'id' => $baseId],
+			['type' => 'user', 'id' => $longerId],
+		];
+		$comment = $this->newComment($mentions);
+
+		$this->commentsManager->expects($this->exactly(2))
+			->method('resolveDisplayName')
+			->willReturnCallback(function ($type, $id) {
+				return $id . ' display name';
+			});
+
+		$this->userManager->expects($this->exactly(2))
+			->method('get')
+			->withConsecutive(
+				[$longerId],
+				[$baseId]
+			)
+			->willReturn($this->createMock(IUser::class));
+
+		/** @var Room|MockObject $room */
+		$room = $this->createMock(Room::class);
+		/** @var Participant|MockObject $participant */
+		$participant = $this->createMock(Participant::class);
+		/** @var IL10N|MockObject $l */
+		$l = $this->createMock(IL10N::class);
+		$chatMessage = new Message($room, $participant, $comment, $l);
+		if ($quoted) {
+			$chatMessage->setMessage('Mention to @"' . $baseId . '" and @"' . $longerId . '"', []);
+		} else {
+			$chatMessage->setMessage('Mention to @' . $baseId . ' and @' . $longerId, []);
+		}
+
+		$this->parser->parseMessage($chatMessage);
+
+		$expectedMessageParameters = [
+			'mention-user1' => [
+				'type' => 'user',
+				'id' => $longerId,
+				'name' => $longerId . ' display name'
+			],
+			'mention-user2' => [
+				'type' => 'user',
+				'id' => $baseId,
+				'name' => $baseId . ' display name'
+			],
+		];
+
+		$this->assertEquals('Mention to {mention-user2} and {mention-user1}', $chatMessage->getMessage());
+		$this->assertEquals($expectedMessageParameters, $chatMessage->getMessageParameters());
+	}
+
 	public function testGetRichMessageWithSeveralMentions() {
 		$mentions = [
 			['type'=>'user', 'id'=>'testUser1'],
@@ -249,15 +323,12 @@ public function testGetRichMessageWithNonExistingUserMention() {
 			->with('user', 'testUser')
 			->willReturn('testUser display name');
 
-		$this->userManager->expects($this->at(0))
-			->method('get')
-			->with('me')
-			->willReturn(null);
-
-		$this->userManager->expects($this->at(1))
+		$this->userManager->expects($this->exactly(2))
 			->method('get')
-			->with('testUser')
-			->willReturn($this->createMock(IUser::class));
+			->willReturnMap([
+				['me', null],
+				['testUser', $this->createMock(IUser::class)],
+			]);
 
 		/** @var Room|MockObject $room */
 		$room = $this->createMock(Room::class);