n2gwtl Oh, my mistake. I thought the issue was with forwarding notifications.
In that case, I'm assuming that the apps that are getting notifications later than they should do so via Google's FCM. Make sure Google Play Services' battery usage is set to unrestricted here: Settings > Apps > Sandboxed Google Play > Play Services app info > App battery usage > *tap the text "Allow background usage" > Unrestricted
Google Play Services still has the network permission, correct?
Also, is it possible some apps were installed after you set up sandboxed Google Play? If so, you would have to reinstall the apps (or clear their storage) to have them set up notifications again with FCM.