Item #55853 (also possibly #56376) exists in Adventure Guide but does not drop. All The Things addon creators have removed the item from their addon because multiple players have done hundreds of runs with no drop. Clearly not working as intended and needs to be either put back on the loot table (likely a bug since the dungeon has been reworked for TWing and M+), or removed from the Adventure Guide. The former would be more preferable I think, as no one wants things to become unobtainable in the game.
Item #133251, which is the Timewalking version, seems to be listed as dropping from the wrong boss and might also be bugged, and it would be helpful if this was also addressed.
Thank you from collectors!