The Many Worlds interpretation of a superposed quantum state (that is, a cat that is in mathematical linear combination of aliveness and deadness) implies that as soon as the quantum switch is turned on, two Realities are created, one where the cat is alive and the other where the cat is dead. This is the essence of what it means for time lines to branch, as in the TV show LOKI.

And this is the way that many Physicists try to resolve the apparent logical contradiction in stating that the cat is both alive and dead. A rule of Logic is the Law of the Excluded Middle, which says that the cat can't be both alive and dead at the same time. (Of course, many do not care about logic, which they call classical Logic, and they just dismiss it as an anachronism.)

So they propose that each probable event corresponds to a separate parallel Reality in which each event happens. That is, there is a Reality1 where the cat is alive, and another Reality2 where the cat is dead. And when you open the box, you find out which Reality you are in.

But that implies that in any Reality, the cat is NOT in a superposition of states. In Reality1, the cat is alive, and in Reality2, the cat is dead. And that also contradicts Quantum Theory.

So the Many Worlds interpretation substitutes one logical contradiction for another one that refutes the very validity of Quantum Theory itself.