When I was doing the dungeon for pinnacles, after beating Caiatl and getting the Mission Complete screen, my friend and I were stuck in the nightmare realm. The animation that plays when you teleport was on constant repeat, but it never sent us back to Eris. The chest contents were put into the postmaster, thankfully, but I waited the whole 90 seconds or so for the results screen to show up, just to be safe.
This happened to me after my first solo run. Along side this glitch, the objective for killing Caiatl never fulfilled, which may be why the triumph didn't complete for me.