Rustbowl - Unable to exit Arena on Bossfight

If you fight the boss in Rustbowl before finishing the other rares, an arena will suddenly be created, and you cannot leave the arena.

Either make the entrance an arena when you enter it (and thus the game knows you're in an arena and you can leave it like the other maps) or remove the creation of the arena state upon killing the boss.

This doesn't seem to happen when the boss is the last rare killed.

It happens to me too. I found a way around it though, press alt f4, open the game again and port in, and you will port in to the waypoint nearest the arena.