You already know that your horse is dealing with pain brought on by a poor saddle fit. Is this jumping done in the new saddle? There is no way you will see her jumping correctly while the saddle is sitting down on her withers!!!
She hollows out her back because for her to jump with her head and neck reaching forward, in a proper bascule (look up that word), would cause her shoulders and back to lift up and put MORE pressure on her wither. She is trying to avoid that. Also, the rider gave her no release , jumped ahead of her , had her feet dangerously too far into the stirrup and her toe pointed down, no base of support whatsoever.
I am not a jumper, but even I can see this.
On a mellower note, have you got your new saddle?