A lot of these nikkas are kids or grown ass men still wading in the kiddie pool mentally.
Afeni was a revolutionary, if she believed that Pac got violated by Police, I think telling the truth about the cops involvement in his downfall would take priority over worrying about Pac's image being tarnished because of something that was out of his control.
Pac said it didn't happen but who...especially RIGHT THEN would admit to some shyt like that? The one person you MIGHT admit that to is your mother who has seen her friends be assassinated by police, you wouldn't tell the 'world.', not the time nor place for that when all of this was going down.
20 years after the fact, especially in a time where we are seeing live executions by the police on facebook, you could debate that it would be an important part to the story. You could also debate that even if it were true it doesn't need to make it in the movie. It would have been on Afeni to make that call and she wouldn't have been thinking at the same level of mind the majority of the people who frequent this board think IMO.