If the many-worlds theory is correct, does that mean there are worlds where many-worlds theory is incorrect?

So assuming many-worlds theory is correct, and every possiblity exists in separate universes, does it follow that there are universes where many-worlds theory is incorrect?

Assuming this for a second, from those universes, do the other universes not exist? If I was a crazy philosopher I might postulate that multi-universe theory must be false because the universes where multi-universe theory is true would have been "cancelled out."

But wait a minute, there would still be multiple universes where multi-universe theory is false, but other things are different. Which still doesn't make any sense.

I suspect that the answer is a simple "if multi-universe theory is true, it is true in all universes." But I can't find an instance of anyone even bringing up this question.