Hey! This happens to me all the time, it usually happens when one person goes into the battle and doesn’t talk to an npc or collect something in the room first. You just walk straight in to the battle. If that is what you did then that is probably what happened

Hey! So this happened to my friend and I tonight as well, but it was in Galleries (the dungeon right before Morganthe). I finished the quest, but for some reason my friend (who was present for when each boss was killed like me) didn’t complete the quest. Since this happened to you as well, maybe there was some sort of glitch in Shadow Palace? My friend emailed Kingsisle to see if there was anything they could do so we wouldn’t have to go through Galleries again (it took a lot of time and stress for us lol), so maybe you could do that too? Hope this helps :).