While fighting Torment 4 Echo of Varshan the Mucus slick eggs would disappear on the ground sometime before I could get them. I also did Gregor and was missing shards of agony.
I also saw a mythic unique disappear as well but I could be seeing thing there since it went away quickly.
The mucus slick eggs though were clearly missing since I started with 150 summoning materials and only got 42 eggs.
I was in a party as well and I’m seeing there is a tribute of harmony boss mat bug as well.
Life hack: Leave the room and port to town. If you are away from the items far enough when porting you should get boss materials and legendaries (or better) delivered into the chest. (far enough = items not on screen + a bit extra)
(Not sure how it ends if you leave the loot that is too low on the ground. You might get it despawned? So far I collected my loot / my mystics.)
Edit: Same problem in here: Disappearing mythic items ? So it might not work - maybe the items get “removed” → then the port won’t change a thing.
Yeah I know about the port but alas nothing there either. I was so surprised because at first I thought someone else was summoning before me but I checked with the floor clear and no one around a boom… gone