Same problem if I update My Favorites, clear My Favorites, even uninstall the Backdrops app then reboot the Roku then reinstall the Backdrops app. I can't get rid of the existing Backdrops images in My Favorites!
But... maybe I have some useful information for your development team:
My Favorites is stuck with the same 55 images that I added to My Favorites about 6 months ago. No matter what I do, I cannot permanently get rid of them out of My Favorites, they keep coming back. BUT... if I add a new image to My Favorites, my image count goes up to 56 images as expected. And if I delete that newly added image and exit Backdrops and launch it again, that newly added image does not re-appear and I have the same 55 images. So new images appear to work with add/delete from My Favorites. It's the old images that don't.
So, it may have something to do with images that were added to My Favorites 6 months ago. Perhaps it has something to do with an index key or unique identifier that is no longer valid, and the Backdrops program cannot find the old image in the database to enable/disable flags for the image.
I dunno, just guessing and trying to help troubleshoot this as a developer myself.