Firebase_Analytics not working as expected after upgrade to 9.1.0 (on Android)

Question

After upgrading firebase_analytics from version 8.3.3 to 9.1.0 my dashboard tells me that my active android userbase has declined by about 50%.

I released the new version on 17.march. I marked it with the red circle. I believe it has to do something with analytics not working anymore on some devices. It definitely works on my testing device and some others as I have users with the current version in my analytics. But it seems like it is not working for at least half of my android userbase.

Did anyone else experience something similar? I read the changelog of course. But I couldn't figure out where there should be a problem. My iOS users are not affected by this issue at all.

I am kind of standing in the dark. I would appreciate any idea.

I initialize my Analytics like that:

final FirebaseAnalytics analyticsInstance = FirebaseAnalytics.instance;
final FirebaseAnalyticsObserver analyticsObserver =
  FirebaseAnalyticsObserver(analytics: analyticsInstance);

and later in build

MaterialApp(
      builder: BotToastInit(),
      navigatorObservers: kReleaseMode
          ? <NavigatorObserver>[
              analyticsObserver,
              BotToastNavigatorObserver()
            ]
          : [BotToastNavigatorObserver()],
      initialRoute: '/',
      routes: {
        '/': (context) => HomeView(model),
      },
...