Arbiter of ash coop

I just attempted the boss with my friend we died twice but on the second death I respawned before him, im the host but once I respawned it took me outside the boss room and sent me back to the hideout and the game wouldnt let me attempt it again despite having 4 more attempts this is brutal because i cant find copper citadels and bought the last fragment and now i lost all my fragments and didnt even get to use all my attempts