> one popular interpretation is that the particle exists in all states, i.e. in a pure description of reality. When any quantum system interacts with it, then it becomes entangled with the result of that measurement, branching it into a new universe (edit for clarification: a new world where it was as if it was never a wave, and it was always a particle).

The two-slit experiment contradicts this. You get different results depending on when you perform the observation(s).

So the new world is a world where the particle was originally a wave, and became a particle when it was observed. Not a world where the particle was always a particle.