I think the decision was a complete cop out. From the replay you can see the ball is clearly over the line. And Bartley is on his back alongside the Forest player - doesn’t look like either are impeded as they are both on their ar@es. The problem is (I think) the goal looks wrong as Bartley puts his hand up against his face - but by that stage the ball is already well over the line.

Anyway it is as it is I guess.