Windows 10 and later heavily recompresses wallpapers before using them
which frequently creates obvious artifacts. Even worse, Windows JPEG
implementation loads these images incorrectly, including when viewed in
the system image viewer, which introduces even further artifact banding.
No other JPEG implementations show this extra banding when loading the
recompressed images. These bugs have been present for almost 8 years at
this point. The only known workaround is this registry entry.

https://old.reddit.com/r/Windows10/comments/nyiec7