Explore The temple Bug

I am currently on ACT 4 explore the temple. The issue I am having is one of the commanders on the level goes invulnerable about half way through the fight and it doesn't drop no matter how long or how many tries I do. I have redone the temple twice and each time this happens, Since this room is one I MUST go through I can no longer progress the story line.

for whatever it is worth it is Xipolo, commander and commanders quarters in vaal temple map.