I built a giant zombified piglin farm over the nether roof in a nether wastes biome. The farm has scaffolding from the nether roof up to the farm. I discovered a pair of regular green zombies in the scaffolding of the farm. As far as I'm aware, zombies cannot climb scaffolding. Additionally, there are no nearby nether portals through which they could have entered.
Environment
version 1.17.1 using latest version of optifine. the world is on a realm. the farm involves entity cramming killing using minecarts. the piglins spawn on magma blocks in a netherwastes biome. not sure when they spawned in as i always fly to the top of the farm and i only just discovered them.
You're most likely experiencing MC-14800