I think what has muted the running game is Stephon Diggs and the passing game. Now that the Bills have committed to a dynamic passing offense with many talented players to get the ball, the focus has shifted away from the run game to the point of it suffering from "neglect." Lee Smith and Patrick DiMarco types are sacrificed for players that are bigger assets to the passing game. Singletary and Moss can still be a big part of the 2020 offense, especially the one that the Bills will have entering the playoffs. The game vs. the Pats shows what could be a more featured part of their last 6 games and I would not write them off just yet. Let's see what the coaching staff may have learned during the bye.
As for the missed screen pass, it seemed like just a mistimed throw to a throttled down target who did not adjust very well to the ball placement. It would have hit for something big and deserves more work and more tries.