Regular zombies spawn in scaffolding in the nether

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.