I saved up enough to get a 1500pnts ldon weapon, was all excited, and bam, it killed just about every collection quest.
Seems the proc, in this case the call of wu, which summons a 'friend' to aid in the fight can randomly cause a kill to despawn instantly. It only happens when the weapon procs, and it also occurs in assassinate mish's with boss corpses.
Can you either remove the proc and replace it with something else, like a dot, or rune like buff. Or make the proc something you can buy as an aug if you want to use it type thing.