Quite normal for forums to drop out your log in if you do not tick the 'remember me' box.
Not sure why anybody would expect to stay 'logged in' on a forum if they do not tick the 'remember me' box - I certainly wouldn't.
I can understand why somebody would not want to tick the remember me box - I have been in situations where I would not tick it - but in those circumstances I would not expect to stay 'logged in' anyway.