Zombified piglins turn into normal pigman in "flat" worlds.

I believe this is a bug, but when ever I am in a "flat" world and I want to spawn in zombified piglins, they will instead turn out to be normal pigman. Even when I go into the nether in the "flat" worlds, they still turn to pigman.
I don't understand why this is happening because this doesn't seem to be the case in my "infinite" worlds.

Hopefully this can be fixed, but if not than that will be ok. It's just annoying.