This isn't much of an issue for most, but if anyone was to set up a base on a mushroom island, this harms the very concept of it being a location without any hostile mobs. This should be an easy fix since they have already been removed from the end. I'm guessing it was overlooke...
The logic for zombie reinforcement is in the methodnet.minecraft.entity.monster.Zombie.hurt(DamageSource, float); it contains several checks, includingNaturalSpawner.isSpawnPositionOk(...)which checks for an empty valid spawn block within the world border, andSpawnPlacements.checkSpawnRules(...)whi...