Mega Salamence bug?

When I mega evolve salamence, he doesn’t change his sprite during battle

I just noticed there is actually no sprite for Mega Mence huh

Yes its a known bug and in the bug fixing list

So when the bug does get fixed, will I need to install a new core?

It will most likely be available both as a new core and a patch you can apply to an existing 1.2 core.