Mega Steelix doesn't have a back sprite?

I’m doing an egglocke, and I finally evolved my Onix into a Steelix. Gave him the mega stone the egg came with, and after I got the ability to Mega Evolve from Perfection, I tried it and… it worked, but no back sprite?

Let me clarify that my issue isn’t the controls. My Steelix DOES Mega Evolve. It does the entire animation, and if I go to change my Pokemon, the small sprite from that menu is of Mega Steelix. But the back sprite remains that of a regular Steelix.

I went and looked into the folders, and there is no Mega Steelix sprite. I then checked the wiki, went to the bottom where the sprites are, but there’s no back sprite there either, only the front sprites.

Is there no Mega Steelix sprite done by you guys, or did it somehow not make it into the latest core? Because, another clarification, I downloaded the latest core, so it’s not like things got messed up when replacing folders.