That's because when you're scrolling the screen left, the image is following the shifting of the grayscale dithering mask, so grays get stuck on the same dither pattern for multiple frames. The "proper" way to fix this would be to display each frame three times, so every frame is displayed with all three dithering masks. This should look much nicer, but it would add a lot of time to each frame. The "improper" way to fix this would be to account for the amount the screen has scrolled when shifting the grayscale dithering mask between frames, so the image doesn't follow the mask. I would do this by shifting the grayscale mask right instead of left (or left twice) if the screen has scrolled one pixel to the left since the last frame.

Yeah, do the same thing for scrolling up as scrolling left. You may also have to account for scrolling in both directions at once, in which case I think you'd want down and right to rotate the mask right and all other diagonals to be the normal rotate left.

Keep in mind that if you add up diagonal and vertical scrolling later, maybe calculating how much columns you scrolled might end up being a major hassle, so if you ever go that far, You might better go with the proper method Runer112 mentionned.