How to Change the Background picture at Login screen?

if you are the sole user (not even a guest account) 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 matches your desktop background.

if you are using file vault or have multiple users, the above will not work.  if that's your situation, you will need to wait for apple to address this. it may be worth your time to send feedback directly to apple. Feedback - macOS - Apple. 

Try changing your desktop background image to some other image, shut down and start up again as a test.

MtD

then i'm stumped. that usually works.

you can look through all the pages of Solution for changing Big Sur login wallpaper to custom to see if anything there helps you.

Verify your Guest User is off.

MtD