Zombie despawning

Put the summary of the bug you're having here

I trapped a zombie in a completely unlit area and gave him a name tag so I can use it
Describe what you thought should happen here

What actually happened was...:
When I did so it was there for around 5 -15 minutes, but it despawned  

Steps to Reproduce:
1. I build the Iron farm design for 1.15.2 by youtuber called voltrox 
2. Lead the zombie to where it was supposed to go and gave him a nametag
3. (Not sure about this one) I went around 2 or 3 chunks away and then returned