Sometimes Windows 10 gets into a bad state where the wallpaper won't change anymore, even in the Windows 10 Settings app. There are two ways to fix this if it happens:
Restart your PC. If you'd prefer not needing to do that, try the second method instead.
Restart the explorer.exe process in Task Manager which should reset your wallpaper. There are instructions for how to do this here.
Wallpaper fit not saved with multiple monitors
The Microsoft Store version has a problem where if you change your Windows 10 wallpaper fit setting after installing the app, the settings change will be reverted after the next wallpaper change. (For example, if you had "Fill" selected when you installed the app and then change the setting to "Span", the setting will change back to "Fill".)
To fix this, make sure you have picked the wallpaper fit setting that you want and then reset the app data. (See the instructions here for how to reset a Windows 10 app.) After doing this, you will need to enter your location and select a theme the next time you run the app.
You can’t perform that action at this time.
You signed in with another tab or window. Reload to refresh your session.You signed out in another tab or window. Reload to refresh your session.