Waning Darkness - Can leave without getting quest to complete and have to completely redo it

empalas

Was on my fifth alt through Undermountain and beat Umbraxakar and then used the amulet, talked to Celeste after the cutscene and left through the arch but it didn't complete. What I figured out was that when I used the amulet after beating Umbraxakar I was distracted and just stayed right there in front of the wall where the arch will appear. Celeste had walked up to me. Talked to her. And then the arch went green and I just used it. The problem was after talking with Celeste it says to approach the arch gate but I was already there. I think the quest needs to finish with talking to Celeste or using the arch. Yes I know I could have backed away and probably then got it finish but it didn't and it took another half hour to finish. Debated whether this is a bug or not but figured I'd let someone else figure that out.