Completed iron citadel says it was failed because player 2 died at the end.

I was playing co-op with a friend yesterday when they died right at the end of the bossfight about 5 seconds before it completed. The notification came up for completing the map then the screen went weird when trying to collect the fragments (like the image was being dragged off screen). It wouldn't let me pick anything else up and upon restarting the game as the start menu didn't work the map now says it was failed rather than complete. What makes this worse is that was a sole connection to another part of the map. So not only has the game failed a map that was completed, it also prevented picking up vital items after the boss fight and blocked one of the few remaining routes on my maps.