Zombie with a helmet holding a cobble despawned twice

I am playing hardcore on a server.

I am creating an iron farm and had captured a zombie in a mine cart. The zombie was wearing an iron helmet that I had thrown to them and they were holding a piece of cobble stone. I pushed them up a minecart track and everything looked fine. I then proceeded to create more of the iron farm and I turn back to see the zombie go up in a puff of white smoke.

I am trying to replicate this now on camera.