How do I fix an "Unable to parse JWT" error on Identity Aware Proxy?

Question

I am trying to use a cloud run endpoint through GCP's Identity Aware Proxy and all of a sudden the IAP endpoint started throwing an error:

Invalid IAP credentials: Unable to parse JWT

I am using the extension_google_sign_in_as_googleapis_auth extension to create a Google client out of my existing Google/Firebase login.

The IAP works fine if I access the api with the browser directly (using the same GCP credentials directly as I am logged into the app with)

I am using the following code, which seems to be connecting to the backend: I see the network inspector fire the CORS head and then the call to the endpoint. The first is fine, the second errors with a 401 and the message above in the body.

getIAPAPI(String path) async {
  Uri uri;

  // make sure the Identity Aware Proxy is addressed authenticated
  var _signIn = GoogleSignIn(
    scopes: <String>[CloudIAPApi.cloudPlatformScope],
  );
  await _signIn.signInSilently();

  // create a GCP client
  final _client = await _signIn.authenticatedClient();

  print('fetching ${path} from api');
  try {
    uri = Uri.https('iapapi.example.com', path);
    var response = await _client?.get(uri);
    return response?.body;
  } catch (e) {
    // print errors and pass back an empty json result
    print(e);
    return "{}";
  }
}

Am I doing something wrong in my code? The really weird thing is that it seemed to work a week ago. I don't see anything in the IAP console settings that could help either.