Was just playing through with a party and when we came to the part, where you have to kill 2 guys and champion mob, while Queen is in a bubble, when I encountered a bug which prevented me from finishing the dungeon. When we killed the champion, it would still show as 2/3 mobs killed. We went back and forth, killing them several times, hoping to finally finish the dungeon, but we had no such luck.
TLDR: Last champion mob won’t get counted as being killed which prevents you from finishing the dungeon.