To fix this we had to call this method when starting up the app in OnStart() in App.Xaml.cs and also call this if the user manually changes the theme in the in-app settings
public static void SetActivityColor()
{
#if ANDROID
var activity = Platform.CurrentActivity;
activity?.Window?.DecorView?.SetBackgroundColor(
App.CurrentTheme == AppTheme.Dark ?
Android.Graphics.Color.Black :
Android.Graphics.Color.White);
#endif
}
Unsure if this will be needed for IOS