I hate that the Berenstain bears is the go to Mandela effect. I was obsessed with the Berenstain bears as a kid. Ive read the authors' autobiography, down a sunny dirt road, at least every other year or so since I was 9. It's my favorite book. In the early years of Stan's life chapter, it's discussed how Stan Berenstain's name was mistaken and mispronounced frequently as a kid. One of his teachers growing up refused to believe it was Berenstain saying what kind of name is that and calling him Berenstein the entire year instead. After both Stan and Jan Berenstain's death people are still getting it wrong. It's not a Mandela effect it's people throughout their lives fucking up the name, sometimes on purpose.
Things haven't magically changed since you were a kid-you just now have the ability to prove yourself wrong. Maybe your mom did pronounce it wrong when reading the books to you which caused you to misremember.