Wallpaper isn't saved to backup

I had Pixel 7a with wallpaper selected from Niagara's wallpapers. It was that cat from Anime → Waiting. When I bought new phone, Pixel 10, I restored the phone from Google’s backup which installed Niagara, then I used Niagara's backup/restore function to carry over settings to new phone. Settings were restored correctly and wallpaper was still correct. Buy when I went into theme editor a couple of days later, to change icons, Niagara claimed it can't save the theme because the wallpaper isn't Niagara's own