Hostile mob despawning with nametag

Hello, today I made an iron golem farm on my hardcore world. though I ran into a problem.

the zombie I used to scare the villagers despawned 3 times in a row. I'm 100% sure I named all 3 of them and I even gave some of them a helmet if they didn't have one already.

 

I'm wondering if I can do anything to fix the issue myself and if not I just wanna mention it to let you all know its a problem.