I just decided to test Phosani's Nightmare, it's still bugged. Specifically, what I think is happening is that the game doesn't know which nightmare you're fighting. You don't guarantee maxhit on the parasytes since Phosani currently uses the Parasytes from the normal nightmare. When you die, your items also get sent to the NPC from the normal nightmare.
We've updated the newspost now (16:20) as we realized the first hotfix didn't completely fix the issue. I would recommend to leave the instance and open a new one if you're still experiencing issues. Let me know if issues persist after that.
Edit: Apologies, it seems that there are bugs remaining for a few players now. I have updated the newspost and our social channels accordingly. We're still investigating why this is happening to some players.