Dying after a boss has been defeated in maps bugs the game

During an Iron Citadel I died to a beam from the final boss after he had died. After respawning and re-entering the arena the loot that dropped had been removed from the map and the boss "respawned" causing him to be invisible and still able to attack me while I could not attack him. Sucks that I lost my fragment to this, but i'm more upset about the xp loss as there was no way I could fight back the second time I entered the arena and I could not return to hideout in any way.