It's been a long time since I last did this quest, but I remember that, especially in the first boss, some people accidentally left through the teleport before the boss was killed and they couldn't get back to the boss room right away. So, if I remember correctly, the room is inaccessible for a while after the boss is killed. Now I don't remember if the teleport becomes accessible again after a certain time (2 minutes, perhaps) or only after the boss spawns again.