[Karellen]

Can anyone add them to the TMDB database?

Yes, you can add them. TMDB is an open community database. Create an account and update the show. Unlike the TVDB, almost no shows are locked, but they do have a very active moderator team that checks edits in the background to prevent vandalism which is a different method to what TVDB does by locking shows.

Is this a recurring theme with maybe more obscure shows?

Obscure=Not many viewers. So yes, if the few viewers don't chip in, then they won't be updated. These things don't happen auto-magically, though sometimes it does seem that way