When /gamerule domobspawning false zombie backup still spawns

What I expected to happen was backup zombies wouldn't spawn when domobspawning is false as they are a naturally spawn

What actually happened was i did /gamerule domobspawning false and it still spawn backup zombies when we kill the zombies that we spawning

1.Make it so when /gamerule domobspawning false all naturally spawns will not happen including backup zombies