Hide Loginname suffix option not working

Hi there, I have an instance where no multiple domains per Org (as well as no multiple Orgs) are planned. I may have a conflicting setting now: since i wanted to have usernames, I kept the checkmark in the domainsettings for the suffix. Otherwise it would use the mailaddress as username which I do not want. So then, in order to not confuse my users, I have added the accessibility-checkmark on "Hide Loginname suffix", which works in the preview but the instance still requires username@domainusername@domain or else complains about a not found user.