Receiving 401 when accessing authenticated Google Cloud Function

I am trying to invoke an authenticated HTTP-based cloud function from another cloud function. Let's call them CF1 and CF2 respectively, for the sake of brevity; thus I wish to invoke CF2 from CF1.

Following the example given by the Google Documentation: Authenticating for Invocation, I created a new service account for CF2, and then attached it to CF1 with the roles/cloudfunctions.admin . I downloaded a service key for local testing with Functions Framework, setting it as the Application Default Credentials(ADC); thus CF2 on my local machine connects to CF1 on GCP, authenticating as CF2's service account via ADC.

I have deployed CF1 on Cloud Functions successfully, and was testing whether CF2 on my local machine could reach to CF1 when I was surprised to receive a HTTP 401.

For reference, here is the code in question, which is almost identical to the samples provided by the Google Documentation:

        String serviceUrl = "<cf1-url>";
        GoogleCredentials credentials = GoogleCredentials.getApplicationDefault();

        if (!(credentials instanceof IdTokenProvider)) {
            throw new IllegalArgumentException("Credentials are not an instance of IdTokenProvider.");
        }

        IdTokenCredentials tokenCredential =
                IdTokenCredentials.newBuilder()
                        .setIdTokenProvider((IdTokenProvider) credentials)
                        .setTargetAudience(serviceUrl)
                        .build();

        GenericUrl genericUrl = new GenericUrl(serviceUrl);
        HttpCredentialsAdapter adapter = new HttpCredentialsAdapter(tokenCredential);
        HttpTransport transport = new NetHttpTransport();
        com.google.api.client.http.HttpRequest request = transport.createRequestFactory(adapter).buildGetRequest(genericUrl);
        com.google.api.client.http.HttpResponse response = request.execute();

I tried referring to:

but I was not able to find a solution to my problem from those questions.

Further testing revealed that the identity token generated via the client SDK: tokenCredential.getIdToken().getTokenValue() is different from the GCloud CLI command gcloud auth print-identity-token. I could use the identity token generated by GCloud CLI to directly invoke CF1 (e.g. via Postman/cURL and authenticated as CF2's service account) but not the identity token printed by the client SDK. This was a surprise as I am using CF 2's service account keys as the ADC, and also authorized it for gcloud access via gcloud auth activate-service-account.

It seems to me that there is no issue with the permissions of the service accounts and cloud functions, as I can directly invoke CF1; thus it would appear to be an issue with the code. However, I am unable to determine the cause of the 401 error.