This has happened to me a few times now, on a few different mobs. About 5 minutes ago I killed a veteran who was part of an event. I opened the loot window on him immediately after death, and just as I was about to stuff my pockets he suddenly de-spawned, taking all of the dropped items with him, never to be seen again.
Is this a bug, or intentional that the bodies de-spawn as soon as the event ends? If its intentional… Well then that sucks for me, I guess.