Multiverse theory is based on quantum theory.

Observation of an election of infinite “possibilities” results in collapse of the wave that encapsulates those possibilities into a single outcome.

You can have the electrons from an electron gun start all shooting into one direction based on whether it’s observed or not. And that’s because each electron is equally likely at all times to go in any direction along its waveform of possibilities. But it will never suddenly start firing lead particles or entire dogs.

The universe in which multiverse theory can’t work would be an isolated universe in which every outcome is already observed and quantum theory and waveforms like the ones discussed before can never be observed.

It doesn’t mean multiverse theory isn’t real, it just means that universe can’t ever even grasp the idea because it’s in a situation where it’s like a 1D person trying to understand a 3D world. There’s enough of a gap that it’s feasible that no intelligent life could ever come to the conclusion they’re an outlier.

Being a sample size of 1 is the most difficult hurdle to overcome in science. And in truth we’re also a sample size of 1 until we somehow manage to make contact with other universes.