https://youtu.be/evAVEfXaPu4This happens with all enemies not just Nephilim. but to make it easy to see I found a group of flying enemies. As you can see, when I killed the third Nephilim I got a drop. But hen I flew to where the drop appeared to be, I got nothing. To get the actual item drop, I had to fly down to where the Nephilim's body was laying on the ground.
Apparently the actual item follows gravity, but the image of the item does not.