I've noticed this on some of the Kanto/Johto's I used in RSE. I guess they got lazy to remake all new back sprites again, so they just modified the old ones.
Yes, I did. I didn't notice it at first, but when I was working with a Mew backsprite and then did the Mew Glitch and traded it to Crystal, I noticed. That also may explain why all the backsprites have so much dithering on them, because they mostly just recolored them and added a bit more shading. They're so lazy they couldn't even revamp properly!
I never noticed that either but I don't think we should criticise Nintendo for laziness. Firstly they produced the game that got us all obsessed and on these forums in the first place.
Secondly the game isn't even made by Nintendo, it's Game Freak.