Not gonna throw a fit about it, but I think there's a weird bug that caused this. Upon completion of my solo run and the last time I ran it with a friend last week, there was a strange bug when Caitl was defeated at the end and Eris is supposed to pull you out of the nightmare but we (and I later) weren't pulled out. We got the visual effect of the green "smoke" like when you're normally teleported out, but then we'd still be sitting there in the nightmare and the visual effect would just repeat. I think because I wasn't teleported out it didn't check the box off for completing the dungeon.