I have seen the same thing and have been arguing it with United fans.
When playing on the break or in wide open games it doesn’t really matter which side Sancho plays on because he has the space to use either foot. But against organized defenses he is really good on the left when he can cut inside onto his right foot to dribble, pass, shoot but fairly useless playing on the right. He doesn’t have enough ability/confidence on his left foot to cut inside from the right and be a real danger, which isn’t really a knock as few players can do that well with their weaker foot.
I think if they play him mainly at RW, especially if Wan Bissaka is the fullback supporting him, they are setting him up for a tough initial season. It seems like Rashford might get surgery and be out a few months so maybe he’ll play on the left for a while though.