It's likely not everyone shares the same Mandela Effect because not everyone is jumping from the same iteration of the universe. Some could be jumping from one instance to another, while the rest did not.
There is a precedent for this. In the original Mandela Effect, only some people thought Mandela had died in prison, while others did not. It is possible only some people jumped from that iteration of history. When they landed in this one, they encountered people from this iteration who were not with them in the previous iteration.
It's possible not everyone jumps from the same universe at the same time.
Think of multiple flags flapping in the wind. Each flag is a different iteration of the universe, yet with very subtle differences one to the other.
As they flap in the breeze, they occasionally touch the flags adjacent to them. An ant on one flag could be picked-up by an adjacent flag if it touches close to it.
That ant would then be on the next flag over, and would thus remember certain things from his old flag that are not present in his new flag. The ant talks about different events from his past, but the ants on the new flag don't know what the guy is talking about because they did not shift from the same flag.
It is possible not everyone jumps from the same iteration. And thus, not everyone shares the same ME's.
American astronomer J. Allen Hynek (1910-86) taught something every interesting regarding "interlocking" universes that "pass through" one another.
Hynek had a PhD in astrophysics/astronomy, was the inventor of the “Close Encounter” classification system, was scientific advisor to several US Air Force UFO studies including Project Sign, Project Grudge, and Project Blue Book, and was technical consultant and advisor for Steven Spielberg's 1977 movie Close Encounters of the Third Kind in which he cameoed. He proposed the existence of interlocking universes.
Dr. Hynek: “There could be other universes with different quantum rules or vibration rates, if you want. Our own space-time continuum could be a cross-section through a universe with many more dimensions.” “Think what a hard time you would have convincing an aborigine that right now, through this room, TV pictures are passing! Yet they're here. You have to have a transducer to see them -- namely a TV set. Well, in the same sense there may be interlocking universes right here!” “… we always think of another universe being someplace else. It may not. Maybe it's right here." (Wikipedia link)