Mektul the Forgemaster (Molten Vault) bug

Hi

If you kill Mektul using Lightning Warp, while he's in the molten gold, the molten gold turns solid and the player character is stuck inside of this cooled lava object, unable to loot the quest item or drops which are spawned outside of the cooled lava object.

I tried "respawn at Checkpoint", but then I couldn't get back down to the boss arena to pick up the quest item.