We have tried just about everything we could think of, solo, 2 players, 3 players, 4 players, 5 players, pet classes, tanks, healers, ranged, melee, dps, level 60 up to 70, mythic, mythic hardmode, using the necklace, without the necklace, some people with the necklace, throwing the stuff into the blender, throwing it in front of the blender, throwing it before the interrogation, standing on top of the item, standing near the item, standing far behind the item, standing far in front of the item, standing to either side of the item.
We consulted the Achievement Discord, and after a short debate, the only thing we could conclude is that the stuff we threw is not reacting to the blending the way it is supposed to, for example; it is supposed to throw the blended stuff back at us in form of many swirlies, that does not happen. It is not far fetched to assume that the interaction is bugged, seeing as Hylbrande and So’leah in the same dungeon are also bugged, Sylvanas in Sanctum of Domination is also bugged, the wall in Mists of Tirna Scithe is also bugged, etc. All of which are Shadowlands content.