There's one skeleton that tracks under the grate at the very beginning.
Drop down, kill him off, door will work like a champ. As a tank I always try to chain him up top before he has a chance to go underneath.
In WS I, it's the croc that gets stuck on the opposing grate, enroute to Slimecraw.
It's an almost 7 year old, 5 minute fix they're simply too lazy to do.