We should return something ;-)

src/Protocol/ActivityPub/Processor.php

@@ -690,7 +690,7 @@ class Processor
 	private static function removeImplicitMentionsFromBody($body, array $implicit_mentions)
 	{
 		if (Config::get('system', 'disable_mentions_removal')) {
-			return;
+			return $body;
 		}
 
 		$kept_mentions = [];