Make sure the settings are correctly nested in the config as detailed in the documentation, in Umbraco:CMS:Content.
The LoginBackgroundImage and LoginLogoImage and LoginLogoImageAlternative are according to the documentation “relative to the wwwroot/umbraco path“. So in other words: your images need to be in wwwroot/umbraco or a subfolder to work.
My paths are all relative, it’s the same format as several other sites running <v16. Only thing I can see is that the three images (/umbraco/management/api/v1/security/back-office/graphics/login-logo etc…) come back as a 404. Not sure if that causes this or not.
if i move the images to wwwroot/umbraco/login_images/… they work fine but in older versions I could just target the media folder. I guess more of a feature request really, that and just putting the images in the plugin’s folder