killed the boss at the end of "secrets in the dark" quest and now stuck

I was collecting the quest-required items for rune of power and I had got to the last obelisk of rust and found the boss and right at the end of the fight we exchanged projectiles and killed each other (the boss dies then me) I then responded at the checkpoint next to the obelisk to find that all boss loot has desponded from the zone. looking for possible solutions to fix this.

Same here, looking for a solution, can't restart with a new character too because it's Cruel difficulty