Problem is that even though it appears back under Notification settings, actually the push notifications are not working (despite all the setting are correct to be received). How can Apple let some mayor issue like this one, not fixed?

Did you manage to make it work, after Resetting Privacy? I had your same issues and, even though the App showed up again under Notifications (and everything was set correctly), still I was not receiving notifications. For Whatsapp I solved it just by Deleting and Reinstalling the app. Very annoying. Gmail and Slack are still not working, and I might do the same thing, I mean deleting and reinstalling the app. This is no sense, and very annoying. Apple, what's going on? How is that possible?