this has actually happened to me twice now. it always seems to involve someone being disconnected and theparties wiping. at Atomos, had a player DC, the party ended up wiping, get back to the boss room, the room itself is locked out, as in you can't get back in, and still has the Iron Giants standing there. everybody had to leave the instance. 2nd time on king behemoth, somebody DCd, party wiped.. get back to room, it's locked out, the floor is still electrocuted, 3 tornados are still going, and 2 of the bomb adds are running around attacking the towers.

I'm guessing the game is still 'thinking' that the DC'd player is still in the room, not dead, so therefore it never resets.