The Ket-Zek on Wave 44 completely disappeared while I was in the process of killing it. It still had about half of its health, and no death animation played.
I ran around the arena completely to see if it had spawned anywhere else, but the entire Arena is empty, aside from myself.
The Wave has not progressed to 45, seems that the minigame is now stuck. I've been standing in the game for about 10-15 minutes, just to see if something would happen.