Arbiter of Ash - Loot disappears on Respawn

As the title says, I was duo'ing with a friend and he died on the Arbiter fight, but I completed the fight and the loot dropped. He clicked the respawn button and the arena reset and all of the loot was gone. Grand Regalia included :(

Anything we can do? Seems like an oversight if you defeat the boss with respawns left...