NavigatorObserver methods do not get called by Navigator

Question

I'm trying to implement a NavigatorObserver in order to execute a method each time a new entry is pushed/popped on the Navigation stack.

First of all, here is my Observer:

class NotificationNavigatorObserver extends NavigatorObserver {
  NotificationNavigatorObserver(this._notifProvider);
  final NotificationProvider _notifProvider;

  @override
  void didPush(Route route, Route? previousRoute) {
    super.didPush(route, previousRoute);
    print("didPush()");
    _notifProvider.refreshNotifications().then((value) => null);
  }

  @override
  void didPop(Route route, Route? previousRoute) {
    super.didPop(route, previousRoute);
    print("didPop()");
    _notifProvider.refreshNotifications().then((value) => null);
  }
}

And here is how I implemented it in my main App:

@override
Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        ChangeNotifierProvider(create: (_) => NotificationProvider()),
        ChangeNotifierProvider(create: (_) => ModalProvider()),
        ChangeNotifierProvider(create: (_) => FullScreen()),
      ],
      builder: (context, child) => Builder(
        builder: (context) {
          final notificationProvider = Provider.of<NotificationProvider>(context, listen: false);

          return Consumer<FullScreen>(
              builder: (context, fullScreen, child) => MaterialApp(
                    navigatorObservers: [
                      NotificationNavigatorObserver(notificationProvider)
                    ],
                    ...

The issue is that none of the Observer's methods are called when they should be. The only related event that happens is a "didPush()" printing in the console on app launch. Nothing else.

For more context, this is how the user navigate in this app:

Navigator.push(context, MaterialPageRoute(builder: (context) => SomePage()));

I don't understand what I'm doing wrong. Where should I investigate?