My friend and I were doing the Infinite Archive together. We were in the third arc, fifth stage, first stage when Hilkarax spawned. We were in the ice arena, I've screencapped below. Hilkarax was where the bad circle is when I killed him. My friend was at the x. My friend was unable to loot Hilkarax. My best guess is that he was denied loot because he was physically too far away - something that could happen very frequently when fighting Hilkarax, since players have to spend a lot of time dodging his lethal storms.