Nov 5 2020
Ah, sorry, I overlooked ticket of version. But it's same. Version 3.0.1 support old syntax too, but it no support user names or mediawiki groups in tag accesscontrol. Only access lists. In czech manual 'skupina uživatelů' is equivalent for access list (page with list of users) and 'seznam uživatelů', defined by parameter 'editAllowedUsers' (or 'readOnlyAllowedUsers') is list of users. Array of users with access to page is generated from raw content of target page with users list.
Version 2.x in MediaWiki is deprecate. More before year was extension completly reworked. But this using groups is invalid for both versions. AccessControl not use MediaWiki groups, but access lists (normal mediawiki page), wíth list of usernames. If you use only 'user', try find page 'user' in main namespace. If not exist, list is empty.
Jul 2 2019
May 29 2019
The problem was solved during Hackaton 2019.