Quest Item Not Dropping in Treasures of Utzaal – The Molten Vault

Hello,
I encountered a problem while trying to complete the quest “Treasures of Utzaal – Clear out The Molten Vault.” After defeating the boss, the quest item that is supposed to drop did not appear. I tried repeating the encounter, but the item still doesn’t drop.

Could you please check if this is a bug, or let me know if there is a workaround?
Thank you in advance for your help!