Firebase Delete User who signed it with apple correclty

I have implemented the Sign-In-With-Apple with Firebase. And I also have the functionality to delete a user. This is what I do:

  static Future<bool> deleteUser(BuildContext context) async {
    try {
      await BackendService().deleteUser(
        context,
      );

      await currentUser!.delete(); // <-- this actually deleting the user from Auth

      Provider.of<DataProvider>(context, listen: false).reset();

      return true;
    } on FirebaseException catch (error) {
      print(error.message);
      AlertService.showSnackBar(
        title: 'Fehler',
        description: error.message ?? 'Unbekannter Fehler',
        isSuccess: false,
      );
      return false;
    }
  }

As you can see I delete all the users data and finally the user himself from auth.

But Apple still thinks I am using the App. I can see it inside my Settings:

Also when trying to sign in again with apple, it acts like I already have an account. But I just deleted it and there is nothing inside Firebase that says that I still have that account? How can I completely delete an Apple user from Firebase? What am I missing here?

so... Apple does not provide this service. But for my case, the following workaround is working just fine.

My sign in process:

1. Check if user signed in before

  // Create an `OAuthCredential` from the credential returned by Apple.
  final oauthCredential = OAuthProvider("apple.com").credential(
    idToken: appleCredential.identityToken,
    rawNonce: rawNonce,
  );

  // If you can not access the email property in credential,
  // means that user already signed in with his appleId in the application once before
  bool isAlreadyRegistered = appleCredential.email == null;

Now to the crucial part:

2. sign in user and check if that uid already exists in Firebase

  final UserCredential result =
      await FirebaseAuth.instance.signInWithCredential(
    oauthCredential,
  );

  isAlreadyRegistered = await BackendService.checkIfUserIdExists(
    result.user?.uid ?? '',
  );

checkIfUserIdExists is quite simple as well:

  static Future<bool> checkIfUserIdExists(String userId) async {
    try {
      var collectionRef = FirebaseFirestore.instance.collection(
        BackendKeys.users,
      );

      var doc = await collectionRef.doc(userId).get();
      return doc.exists;
    } on FirebaseException catch (e) {
      return false;
    }
  }