Boss arena exit missing in Underground Sea

After killing the boss in Underground Sea map and moving to the left to collect the loot, the boss room exit will disappear. The only way to get back into the map is to exit to Character Selection screen. If I try to re-enter the arena, the exit will still be gone. This has happened in 4 out of the 5 Underground Sea maps I have played today.


edit: This maybe only happens when you kill the boss offscreen.