Named Zombie Despawning

So I'm aware that villagers need to sleep and visit their work stations to re enter panic mode for iron farms, but whenever I put a named zombie into the farm he despawns after a couple of minutes.  I am one hundred percent sure I named him, as I've used 5 nametags on multiple zombies.  I'm not sure what the issue is here because I thought named mobs couldn't despawn so I was wondering if this was a bug.