I finally found a solution on a macrumors thread and thought I would share here (first time posting so hopefully I'm within the guidelines of linking to an outside webpage - I wanted to credit the poster for the solution)..
https://forums.macrumors.com/threads/photo-albums-background-another-ios18-blunder-solution.2456456/
Seems it happens when Reduce Transparency is turned on while using Dark Mode, but there is a way to turn it off for just the Photos App.
Basically.. go to Settings > Accessibility,
scroll down and select > Per-App Settings,
select > Add App, then scroll down and select the Photos App,
then Select Photos (on the Per-App-Settings page) and turn OFF Reduce Transparency
It worked for me. I had been taking screenshots of a black screen to put in albums, but I had to repeat that often for the screenshots album to keep it at the top. It was worth it, though kind of a pain.. so really glad to have a better solution!
j