Hi Johnny,
This may be interesting as a workaround.
I found out that if I create a new user account (Can be a local account) then the Spotlight collection for the Background wallpaper populates pretty quickly (within 30 minutes) after selecting Spotlight.
After I saw that, I deleted the original user account that lost the collection and recreated it, I got back the collection in the next 30 minutes.
From what I saw, this feature seems to work correctly only on newly created user accounts not on accounts that existed before upgrading to 22518. 🤔
Edit:
As of 12/13/2021 the new User accounts created after updating to 22518, have maintained the Spotlight Collection for the Background picture. We'll see if they last or if it is just another coincidence. 😎