SubscribeToTopic() results in 401

[metodiobetsanov]

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• For issues related to the code in this repository file a Github issue.
• For general technical questions, post a question on StackOverflow
  with the firebase tag.
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  google group.
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  of the above categories, reach out to the personalized
  Firebase support channel.

[REQUIRED] Step 2: Describe your environment

• Operating System version: Ubuntu 22.04.5 LTS
• Firebase SDK version: 3.1.0
• Firebase Product: Firebase Cloud Messaging
• .NET version: 8.0.10
• OS: Ubuntu

[REQUIRED] Step 3: Describe the problem

We have 2 projects on GCP, project A is used to host workloads on GKE, and project B is set with Firebase
We have a service/api that will handle notifications requests from client apps and will send them to FCM.
The authentication is done via ACD and workload federation, the SAs are given all permissions required.
We are able to send notification and data messages to the devices, but when we send a subscribe to topic request we get 401

Steps to reproduce:

1. intiialize the default instace for the app,
2. get the messaging service
3. use SubscribeToTopicAsync to subscribe a device/s to topic

Relevant Code:

FirebaseApp app = FirebaseApp.Create(
new AppOptions
    {
        ProjectId = firebase_project_id,
        Credential = GoogleCredential.GetApplicationDefault(),
    });
var firebaseMessaging= FirebaseMessaging.GetMessaging(app);

TopicManagementResponse response = firebaseMessaging
                                                    .SubscribeToTopicAsync(new[] { "device_token" }, "test_topic")
                                                    .GetAwaiter()
                                                    .GetResult();

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