I'm surprised no one's mentioned (either the perception of or the reality of) #MeToo overreach and resulting backlash, which was BLM backlash two or three years before BLM backlash was BLM backlash. Left-wing sociocultural movements really need to start accounting for this pattern in their strategy.
It's just harder to have any form of message discipline in the social media era. We're watching this in real time with the pro-Palestine left. Social media is designed to elevate increasingly radical takes. Some of these get adopted and absorbed into the mainstream of the movement, but those that don't still get outsized attention in a way that paints the entire movement negatively. I'm not sure what the solution is, although getting teenagers off social media would be a good first step.