The splash screen does not use the SDDM wallpaper, or any other (configurable) wallpaper. This would be useful, to prevent the jarring screen changes, and makes the setup more uniform. Reproducible: Always
Yeah, I notice that 99% of splash screen themes on store.kde.org are simply the default one with a different background, which is slightly silly. We could probably make it much easier for people to self-satisfy by making the background user configurable within the theme or even KSplash itself