Live wallpaper turns black after animating

Which animated wallpaper are you using when this happens, or does this happen regardless of which you use?

Live wallpaper animating on the lock screen, then going completely black after unlocking—isn’t normal, and it’s likely a bug with how macOS handles animated wallpapers in certain scenarios, especially on the iMac with its integrated GPU setup. It seems like when the system transitions from the lock screen back to the desktop, it’s failing to reload or maintain the animated layer, leaving you with a black background.

One thing you can try, regardless of which animated wallpaper you use is to disable "Reduce Motion" (if enabled) in System Settings > Accessibility > Display. Make sure “Reduce motion” is turned off. That setting can sometimes mess with animated content.