Which MB model? Which version of Monterey?
Depending on the latter, try:
- Set System Preferences>Security & Privacy>FileVault>Turn Off FileVault
- Desktop & Screen Saver>Change background to background of your choosing, or change to another and change back
- Apple menu > Lock screen >> background should be the one you've chosen
- if you are the sole user AND file vault is turned off you can go to System Preferences>Users & Groups >Login Options and set "Display login window as" to "List of users". Then your login screen should match your desktop background.
- Turn FileVault back on