Put the summary of the bug you're having here

What I expected to happen was...:
minecraft to be normal and for no abnormalities to uprise.

What actually happened was...:
millions of mobs, i run /kill @e[type=bat] and it kills up to 300 at once.
spam it and it keeps killing groups of 5-20.
plus, this, with mobspawning off by the way, and all naturally spawned:

Steps to Reproduce:
just play minecraft.

any way you want to, and then check by running /kill commands and seeing ow quickly and how many of one type of mob spawns. bats are the best example in my experience.