The reason behind this is quite simple, the Spotify application which is downloaded from the app store comprises of a signature different from the Premium Apk. Addition of premium disables the checking of signature and writes a different signature. Spotify is a top global grossing application on every app store and the anti-virus is registered with signatures of the Spotify version downloaded from the app store. Thus, anti-virus simply alerts you that the App does not belong to the app store. You can discard this as you are aware that the Spotify APK is unique and comprises of premium.