When I start typing a user's realname, it correctly suggests that user (by their realname. When the user is selected, their username is entered and highlighted. When the comment is posted and appears in the newsfeed, their name appears as [@username:userIDnumber], for example [@donald:7].

Everything is up to date and I've disabled every other plugin to try and get it working (I've since enabled some again but the problem persists).

Please tell me how to fix the issue or arrange a refund. Thanks.