The Beachhead Boss not Spawning

I'd been saving a Beachhead map for when the challenge bug was fixed, and hopped on this morning. I leapslammed all the way to the arena immediately and went inside, only to find that nothing was spawning from the giant portal gate. I don't know whether that part is 100% a bug, given that I killed nothing else in the map and am unaware if something triggers the boss encounter, but I have an active prophecy which could be interfering.





More annoyingly though, given that I opened a portal before I went up and saw the bug, is that the exit to the arena has closed. I cannot leave the arena and now the map is wasted -- going back to my hideout and coming back in doesn't help.