MyBB Community Forums

When I add a user who is in the default Members group as Moderator of a specific forum, he automatically joins the Moderators group. 
But when I remove him from moderating a specific forum he remains in the Moderator group.

My suggestion is: when you remove the user from moderating a specific forum and he is not moderating any other forum he goes back to the old Primary Group he belonged to.

You are right, I think this might be a bug.

Edit: I think a possible solution would be to store the original GID in the moderators table and restore it when deleting the moderator.