Drilgann mega evolution isn't complete

So when I mega evolve my Drilgann, my pokemon don't evolve but she get the stats of her Mega evolution.

Backsprites aren't ready yet

OKay Sir

Don't call me sir pls, I'm only 14

If you check Drilgann's Status Screen you will see that its Mega Evolution's front sprite is there. For whatever reason, its backsprite is missing -- although it is complete, it didn't get added right.

And Shiny S51-A's backsprite is gone too while his Shiny Mega backsprite is there