Mobs spawn in deep dark biome

In the deep dark biome, mobs can spawn there.  This is unintentional.  I have encountered about 10 skeletons and two creepers, not to mention zombies.  Whilst they cannot spawn a warden, they made us jump and move by hitting us, creating noise and spawning not one but two wardens.  To confirm, we were in a deep dark biome as there was sculk around.