zombie pigmen are not despawning

Whenever i go to my pigmen farm, no pigmen are spawning, so i checked around and i found a lot of pigmen near my wither farm, so i trapped a few, set my render distance to 2, walked about 500 blocks away to unload the chunks, and when back. The pigmen i had trapped were still there

What I expected to happen was...:
The pigmen should have despawned. im pretty sure that pigmen despawn. right?

What actually happened was...:
The pigmen were apparently not despawning even when i had walked a long distance away from them

Steps to Reproduce:
1.trap pigmen
2. walk a long ways away
3.go back and they are still there