Listening to a new document does not work.

[MauroGentile]

In the init of a Flutter GetxController I call these 2 methods:
fetchUserData();
listenUserInfo();
defined as

 Future<void> fetchUserData() async {
    UserModel userData = await _firebaseDB.getUser(_authController.userId!);
    user.value = userData;
  }

void listenUserInfo() {
    _userListener =
        _firebaseDB.listenToUserDocumentChanges(_authController.userId!).listen(
      (DocumentSnapshot<Map<String, dynamic>> snapshot) {
        if (snapshot.exists) {
          UserModel updatedUser = UserModel.fromDocumentSnapshot(snapshot);
          user.value = updatedUser;
          print("User data changed: ${user.value.id}");
          print("User data changed: ${user.value.name}");
          print("User data changed: ${user.value.email}");
        }
      },
      onError: (dynamic error) {
        print("Error listening to user document changes: $error");
      },
    );
  }

The listener above works fine on users already existing in Firestore but fails to listen to changes if the document has just been created.
In other words, if I launch the application and register a new user, the corresponding document is automatically created in Firestore. Then I set up the listener on the document just created, but changes are not detected.
To make it work, after creating the user, I have to restart the application. Alternatively, I have to introduce a delay of roughly 1 sec but either of teh above solutions is not really convenient in production.
I assume this is not a bug. I guess there is something I am missing in my code. Guidance would be very much appreciated