Crimson / Fire ants sometimes spawn as normal zombies

I created an ant farming system in the nether (overworld / building world) which quickly kills ants and also causes them to spawn back in quicker but I’ve noticed normal zombies start spawning in too.
Normal zombies aren’t supposed to spawn in the nether, and being that I know the ants have some zombie ai in them or are a possible reskin I’m assuming its some kind of bug with that which causes some to spawn in as zombies instead of ants.
Or at the very least the ants are causing zombies to somehow spawn in in the nether in a way I’ve never seen before when I activate my farm.