Unable to set Payload/Body on Google Cloud Tasks

I'm trying to use Google Cloud Tasks, creating a task via "Try this API" web feature or the Ruby Google Cloud SDK.

I can't get the payload to be delivered to a worker.

1. Sending the Http Method as POST actually works, but shows up as GET in the Cloud Tasks UI.

2. No payload or headers are sent to the worker or shows up in the Cloud Task UI. I've tried Base64, JSON, normal strings. (see images below)

Example:

Request:

{
  "task": {
    "httpRequest": {
      "httpMethod": "POST",
      "body": "SGVsbG8=",
      "url": "https://some-random-url"
    }
  }
}

Response:

{
  "name": "projects/<my-project>/locations/europe-west1/queues/default/tasks/73572672049866200001",
  "scheduleTime": "2019-07-03T14:49:34.450453Z",
  "createTime": "2019-07-03T14:49:34Z",
  "view": "BASIC",
  "httpRequest": {
    "url": "https://some-random-url/",
    "httpMethod": "POST",
    "headers": {
      "User-Agent": "Google-Cloud-Tasks"
    }
  },
  "dispatchDeadline": "600s"
}

Example Ruby code:

task = {
          http_request: {
            url: <project/location/queue/url>,
            http_method: 'POST',
            body: "Something"
          }
        }

        response = @client.create_task(@parent, task)

Screenshots from Google Cloud UI:

Empty headers

Empty payload

Is there some special format to set a payload to be delivered via POST ?

Thank you!