There are some problems with Zombies, Zombie Pigmen, Armor, and Skeletons, it’s that they have player forms, which means they’re not supposed to be mirrored, and they can have overlays! Please fix this soon, I need it for my resource pack.
Just because they are player based does not mean they have to change. If they were to change it would not effect gameplay that much to begin with, so what is the point?
Hum, did they made the overlay changes for Biped subclasses too? I am not aware of this.
Also, I tried to change the zombie.png skin to a player skin that have an overlay, and the overlay does not shown.
This is a minecraft limitation, not the editor fault. Zombies cannot have unmirrored arms or overlays.