In case it is created automatically, it needs to be updated automatically when a team is promoted/relegated.
There is no sense of having a folder that does not have use.
If you consider the intent here, it is probably to give a new user easy access to their league forum which they otherwise might be slower to find. Many users wish to continue communicating with their old league buddies after promotion/relegation, so removing a forum from the favourites by default is not a desirable feature. No reason to consider it as a bug.
Perhaps it would be good to automatically add the new league's forum to favourites. I would however imagine the expectation is that those users who use the forums have by the time of their first promotion/relegation learnt to use the forums.