Its likely still mutter, but waylands forced vsync and compositing likely resolves it. X11 can have issues with vsync on some systems among other issues in regards to tearing which wayland simply doesnt have.
As for GNOME devs fixing it, youll have to prove without a doubt its mutter and nothing else or nothing will happen towards fixing it so as to not waste time.
I see, pretty sad the devs act like that, i thought gnome community is kinda more people-oriented, still thanks for your time, you explained a lot of things to me 
Well while GNOME devs can be a bit terse, the project is large and has a lot of potential issues which need attention. If they cant be 100% sure it needs them to fix it they will pass the buck. This is the case for other OSS projects as well so as to not bleed resources on every single issue that crops up unless you can readily pin down the issue.
GNOME community has its faults, but this case they likely just dont know what the cause is and if its worth their time which is finite.
keep up on the issue and if they ask for testing see if you can do so to resolve things.
I think they will forget it, what DE do you use? 
I used GNOME for a long time but KDE Plasma atm and Plasma Beta specifically right now.
If you want you can check out my making Plasma more cozy for GNOME users thread
The only reason i dont like kde - application style, they feel so cheap… And it’s so hard to find a good one. But… now i found around 3-4 wayland bugs which are even more annoying than the tearing so ill try KDE and switch back to gnome in year XD
feel free to post in the other thread, perhaps we can find you a nice style
I’m sorry but AhHaHaHaHaHaHaHa
Its a matter of personal preference, no need to be like that
Though i will say I think breeze is a pretty great default theme as i have a deep hatred for the colors of adwaita (though i like the shape/shading)