Affects version: 1.15.1, possibly sooner.

How to reproduce:

1. Spawn baby zombie riding a chicken.
2. Demount / Dismount baby zombie (by spawning it in an ocean biome maybe?)
3. Leave the remaining chicken / move away by a couple of chunks
4. The chicken will despawn eventually which should NOT happen. 

Fix:

Chickens spawned in when a baby zombie rider is spawned and killed should not despawn in the world, like how normal animals don't despawn.

Notes

I found this to be happening when I was playing in an ocean biome. I found these chickens in the middle of the ocean, took them to my island. Eventually they despawned and did not produce any eggs.