[firebase_dynamic_links] getInitialLink() works and logs in but onLink() doesn't log in

[FickleLife]

I have some code:

// vars
  String _email; //this is set earlier on when user logs in
  String _link;

// handle all dynamic links
Future<void> retrieveDynamicLink() async {
    FirebaseDynamicLinks.instance.onLink(onSuccess: (PendingDynamicLinkData dynamicLink) async {
      final Uri deepLink = dynamicLink?.link;

      if (deepLink != null) {
        _link = deepLink.toString();
        print(_link);
        await _signInWithEmailAndLink();
      }
    }, onError: (OnLinkErrorException e) async {
      print('onLinkError');
      print(e.message);
    });
    final PendingDynamicLinkData data = await FirebaseDynamicLinks.instance.getInitialLink();
    final Uri deepLink = data?.link;

    if (deepLink != null) {
      _link = deepLink.toString();
      print(_link);
      await _signInWithEmailAndLink();
    }
  }

// sign in with magic link
  Future<void> _signInWithEmailAndLink() async {
    bool validLink = _firebaseAuth.isSignInWithEmailLink(_link);
    if (validLink) {
      try {
        await _firebaseAuth.signInWithEmailLink(email: _email, emailLink: _link);
      } catch (e) {
        print(e);
        _dialogService.showDialog(
          title: 'Error',
          description: e.toString(),
        );
      }
    }
  }

The first function works in so far as it correctly detects an getInitialLink() and onLink() - I can see it printing out different deepLinks correctly. getInitialLink() path correctly logs in the user, however the onLink() path does not, saying that [firebase_auth/invalid-action-code] The action code is invalid. This can happen if the code is malformed, expired, or has already been used. The deeplink is unique on this path, so I can't see why it wouldn't log the user in.

Am I doing something obviously wrong here?

flutter doctor:

✓] Flutter (Channel stable, 1.22.5, on Mac OS X 10.15.7 19H15 darwin-x64, locale en-GB)
    • Flutter version 1.22.5 at /Applications/flutter
    • Framework revision 7891006299 (4 weeks ago), 2020-12-10 11:54:40 -0800
    • Engine revision ae90085a84
    • Dart version 2.10.4

 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/alexblack/Library/Android/sdk
    • Platform android-29, build-tools 29.0.2
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 12.2)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.2, Build version 12B45b
    • CocoaPods version 1.10.0

[!] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)

[✓] VS Code (version 1.52.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.18.0

[FickleLife]

I've tracked it down to #2132 - FirebaseDynamicLinks.instance.onLink(onSuccess: is firing twice.... wonder why it's happening on Flutter 1.22.5 when a fix was applied channel dev 1.15.21...