I have figured out what causes my experiences with the poofing loot bags.
It has to do with the monsters only dropping gold (no item)
When a mob drops just gold, it drops a loot bag real fast but gets picked up and often goes unseen.
When you are fighting multiple monsters and the first/second (any but final) monster drops gold, they spawn a loot bag, but when you kill the final monster, the gold bags poof and dissappear. This can be seen easier with the loot setting to Dialogue for items but not for gold (notification otherwise)
It is probably supposed to be this way, but it sure causes some paranoia when grinding for items.
Taking screenshots now, ill post them in a few