It appears that update to Monterey 12.7 (Sept. 22, 2023) has corrected this issue. Has anyone else found that to be true?
I have tried many of the suggested fixes but none worked. Today after yesterdays update I chose one of the other standard photos in System Preferences Desktop & Screen Saver and on reboot it was on my Macbook Screen and external monitor as well both on the desktop and login screen. Then I chose under colors the plain black that I have had and lost and wanted to restore as both desktop background and login screen background. On reboot...the black was in both places as desired!
MacBook Pro 13″, macOS 12.7