1. I have two 24' monitors. One is 4K and the other is 1080p. Displayfusion thinks the 4K one is larger than 1080p one, which makes the combined wallpaper does not work properly as the wallpaper in 1080p monitor being scaled to a small region of the wallpaper. (screenshot attached) Because both monitors are in the same size, the scale makes the wallpapers on two monitors are not connected correctly.

2. Almost all the wallpaper sources lack the functionality of filtering only 4K wallpapers. As I know, the wallpaper abyss has the function of filtering by resolution, which is also shown in the displayfusion's settings, but it is not functional. Even I fill the resolution as 3840x2160 (as in screenshot), the search results are still full of 1920x1080 or smaller wallpapers. (the resolution filter does work in wallpaper abyss website)

Oh, by the way, because my monitors have different DPI setting, the tasker bar on 1080p monitor has very small font. (and it has different font family as the real windows taskbar)

Although I can adjust the ratio in the adjustments tab, it seems that the cropping happens before the ratio applied, see the screen shot.

For Wallpaper Abyss, I can still reproduce the problem. See the second screenshot. Most wallpapers in that search result are smaller than 4K.