This is a puzzling phenomenon that many people report as having experienced, whereby 'history' as accepted now, differs from what they personally recall.
It is given the term 'Mandela effect' because of the very many people who reported that they recall Nelson Mandela's death being reported decades before the 'recent' death only a few years ago.
This has also been the experience applied to several other deaths of famous/notorious persons whose demise has received public attention.
The phenomenon not only applies to such deaths, but to strange changes between what is remembered and what is now regarded as 'fact'.
For example -
..A man reported that during a regular drive past a row of shops, he noticed one has changed type of trading overnight and the shop owners insist they have traded those goods there for years.
These anomalies represent subtle puzzling changes in what is accepted as 'reality'
Another frequently reported anomaly is the location of New Zealand's position and distance from Australia. - It isn't where I recall school maps showed it in the 50's.
It is evident that 'historic memories' differ between people.
Persons of all ages report it, so it isn't 'dementia related'.
One possible theory for it is multiple parallel universes coexist which have subtle insignificant changes (glitches) from each other, where a 'you' exists in all of them, and your conscious awareness has unintentionally shifted from one to another.
I have personally been puzzled by several such anomalies, and hope you can tell some of your own (I don't want to be the only nutter is the village
)
In the 50's I recall reading newspaper headlines reporting the fire destruction of the Crystal Palace. I believed this to be a 'fact' which to me was incidental and irrelevant, and was puzzled to discover, only a few years ago, that it happened in 1936 - before I was born !