On my superflat world I was messing around with different enchantments on swords, and was using zombies as test subjects. I soon started seeing zombies swarm the other side of the caged in area of the fence. I then turned off mob spawning, and sure enough they kept spawning. I believe the problem is that new rule where zombies have a chance of spawning other zombies after they're killed, and someone forgot to change that when doMobSpawning = false