It seems that there’s no back sprite for mega lopunny in the Graphics–>Battlers folder
(I checked the zip file for the latest core release and its also missing)
ah thank you for letting us know. seems we missed this one also in the lastest update. if we have time we will have a backsprite for mega lopunny in the next patch. along with mega sableye and salamance.
Mega Beedrill also has no back sprite.
Add Mega Salamence to that list