I have our LDAP server configured in User Directories. It seems that the OU defined group allows users to login with their LDAP credentials, but users don't appear in the People Directory. When tagging other users @username sometimes appears for one or two users (we have a small team of 9) but eventually, when we are able to tag the @username gets replaced with @Unknown User (email@address). Same thing with articles that were written by people who were allowed to log in and have since disappeared.
Are the users not being added as users once they login with LDAP credentials? It's like they're being allowed in, the users are allowed to work, but then confluence forgets about them.