Wallpapers not shuffling properly

MacOS 26.2 doesn't shuffle the built-in wallpapers for me. I downloaded some landscapes (even made sure the files appear in the file system) and set the wallpaper to "Shuffle Landscape" and tried different intervals (continuously, 12 hours, and 24 hours), but it always stays the same. I also have it synched to the screensaver if it should matter. Please help me out here!