I can't speak for everyone, since my memory is different even from the consensus memory of other people who remember his dying in prison, but I can tell you what I "remember," and why it may not be as absurd as you think.
I misremember his dying in prison in 1993. In my false memory, he never became Prime Minister. His death inspired the movement that finally ended Apartheid, but he didn't do it himself. His death was interpreted as a kind of martyrdom for his cause, and as the deaths of martyrs often does, it evoked sympathy even in the hearts of people who disagreed with him, eventually leading to the end of Apartheid.
I was only 7 in 1993, so I'm more than willing to accept that I'm just misremembering completely. My geopolitical awareness in the 2nd grade was pitiful by the most generous interpretation. Perhaps I'm conflating lots of different pieces of other stories and mistakenly associating them with his name. My memory is generally quite reliable, in my experience, but it's a long way from foolproof. I do remember being genuinely surprised when he actually died, long before I heard of the "Mandela Effect," because I thought I could remember the lesson in 2nd grade when we learned of his death in prison.
I just wanted to mention it's a whole different set of memories, including his position and role in ending Apartheid, that I misremember, not just the circumstances of his death, and I would guess that's true of others as well.
At least in my mismemory, he was still heroic, and his life (and death) inspired the change he's remembered for now. I would hope not to insult anyone just because my memories of my ignorance as a small child don't match what actually happened.