That I absolutely agree with!
As for exactly when it should be ‘turned on’ for effect? The moment a character has finished the complete campaign present at the moment.
They are not meant for that, by the time you reach em you won’t have a key, so you turn back anyway. I get what you wanna say with that… but this part is actually intentionally designed like that, to keep the place in mind for the player.
Kinda works, a bit.
Not optimal but at least it works 
Fair!