for me it seems to happen to all mobs. for when I die and go back to the spot I died, the mobs that killed me seem to go invisible. it doesn't happen all the time, but when it does I just need to relog and they're back.
it also sometimes happens that mobs become ghosts after I die and go back to that spot. they are there and I can see them, and they damage me, but I can't hit them. but the same with this, relog seems to fix it.