Unknown Cause, Possibly leaving and entering the world. Can't interact with Ghost Item Frames . F3+B hitbox still shows. If you aim for the entity the block behind it's actualy destroyed. They still drop as an item if there's no block behind it. Tested in all gamemodes, you can still spectate through their point of view though.
Update: This may be caused by the game calculating the hitbox inside the block it hangs on.