I do not " believe in" things as if I were some religious nut bag. I accept the evidence for phenomenon if substantive and also having many lines of convergence on the phenomenon.
The "Mandela effect" has not supporting evidence for its assertions of parallel universes and variance in human memories aside from the actual facts of a matter. This "effect" is just a subset of the psychological concept of confabulation, the concept dealing with these erronious memories.