Last wave of the last round isn’t spawning. Torgue screamed about a boss fight, but they just aren’t here, and the door to leave isn’t opening. There are no enemies in the arena.
This is happening on my second run through the Slaughterstar (I finished one, then spoke to the Atlas soldier to start another one). I noticed throughout the second run that new rounds and waves seemed to be announced before I had finished whatever one we were really on.
That used to happen sometimes when I would die and restart, but then the whole mission would at least still go to completion.