Hmm. I tried updating the profile image in System Preferences > Apple ID (and also from my iPhone's Apple ID settings). I'm having the same experience as you - that image seems to update everywhere except Mac App Store. (The iOS App Store displays the currently-selected Apple ID profile image.)
I have to assume it's meant to update from the image we select for our Apple ID, as the image clears to the default profile icon if you sign out of App Store. (Yes, I tried a sign-out/sign-in, but the old image returned when I signed back in). I'm confident the image is not pulled from Users & Groups, as the icon I use there has never appeared anywhere other than on the Mac login screen.
I've also held the Shift key while re-opening App Store - some apps (Safari is a prominent example) use Shift-Open to Clear Stored State. It didn't help.
So I'd assume there's something broken. Since bug reports tend to get attention based on how many people report the same bug, I wonder how many would bother to report this minor annoyance?