The item/mob dissapears, along with the hit box. (Noticed it with horses, item frames, and boats). Collision detection and the sound, in the horses case, still frunction. Blocks will disapear the moment you place them. Was able to fix item frames by breaking the block behind the frames, and by pushing the horse around I was able to make it reappear.
Duplicate of MC-139122 (I'm assuming the horse and item frames started to disappear after you died once)