I was recently building an Iron farm but everytime I caught a zombie villager and made it hold an item and wear helmet, it eventually disappeared. There were 3 instances of the same happening, my observations of which are :
1. 1st Zombie was wearing a player given helmet and sitting in a boat. I never even left the loaded chunks but was busy with the farm. When I looked to the zombie after a interval of about 15 minutes, I saw the boat and zombie flesh. No Wolves/Goats/Golems are in the area.
2. 2nd zombie was wearing full chainmail armor, got him inside the zombie chamber of the farm. then golems proceeded to spawn in the Golem chamber indicating the farm was working when the zombie disappeared.
3. 3rd zombie was in a boat with stone in hand wearing iron helmet. despawned from the boat when I was less than 20 blocks away from him. Only remains in this case was the stone he was holding, no rotten flesh was found.