Mobs going invisible when dying

I'm creating a medevil-like map, and I was testing my knight gear, I died and, noticed nothing wrong, because I wasn't paying attention, I went to /gamemode 0, and noticed I was being hit, I did /kill @e[type=Zombie,r=3] and the chat Outputted: "Killed Zombie", I summoned another zombie and did /kill, looked to where I died and the zombie wasn't there, neither the slimes, I know the zombie was there, because I did /kill @e[type=Zombie,r=3] near where I landed, and it said the same thing.

This happens if you die, all entities seem to go invisible, including items.

I have a video, for proof of this bug, I looked around the site and I didn't see the bug anywhere, so sorry if it's a dupe!