Android device group not receiving messages

The official reply I've gotten from Firebase is: they simply don't support the no-server configuration anymore.

So after 30 emails with Firebase support here's the surprise: there's nothing wrong with my code/project, and the funny thing is that they don't know why and how it worked before! Unfortunately, they didn't advise anyone about this big change, so any app based on Android device groups without a server doesn't work anymore.