More accurately, an established phenomenon was given a definition that linked it to a what was indeed "lapse in memory" re: Mandela, helping to downplay every other instance of the phenomenon that has unfortunately been named after Mandela... some people will still insist that Mandela died in the 80s. I cannot personally speak to having any memories from the time, as I was not yet alive. My post 1980s memory has always been that Mandela was alive, but again, I can't speak to what may or may not have been said in the 1980s, and it would seem that some accounts indicate that there was a "rumor" that Mandela died, in the a similar manner to rumors about celebrities dying persist today... how many times was it reported that Betty White died?

But again, the "established definition" itself, is a fake definition, as it does not accurately describe the phenomenon. Unfortunately because of its common usage, the phenomenon and the name have stuck, to make it "easier" to communicate...

"Mandela Effect" is in the same category as "conspiracy" and "Qanon"