Hubspot API

I make a request to POST https://api.hubspot.com/automation/v4/actions/{appID} with the following body
{
  "actionUrl": "https://test.com",
  "published": true,
  "outputFields": [
    {
      "typeDefinition": {
        "name": "testValue",
        "type": "number"
      }
    }
  ],
  "labels": {
    "en": {
      "actionName": "Test action name",
      "actionDescription": "Test action description",
      "outputFieldLabels": {
        "testValue": "Test Value Label"
      }
    }
  }
}

The respone is a 201 with this in body:
{
    "actionUrl": "https://test.com",
    "published": true,
    "inputFields": [],
    "objectRequestOptions": null,
    "labels": {
        "en": {
            "outputFieldLabels": {
                "testValue": "Test Value Label"
            },
            "actionName": "Test action name",
            "actionDescription": "Test action description"
        }
    },
    "objectTypes": [],
    "outputFields": [
        {
            "typeDefinition": {
                "name": "testValue",
                "type": "number",
                "fieldType": null,
                "options": [],
                "optionsUrl": null,
                "referencedObjectType": null,
                "externalOptions": false,
                "externalOptionsReferenceType": null
            }
        }
    ],
    "id": "31903326",
    "revisionId": "1",
    "functions": []
}

This includes 'labels.en.outputFieldLabels'.

Then making a request to GET https://api.hubspot.com/automation/v4/actions/{appID}/31903326 returns
{
    "actionUrl": "https://test.com",
    "published": true,
    "inputFields": [],
    "objectRequestOptions": null,
    "labels": {
        "en": {
            "actionName": "Test action name",
            "actionDescription": "Test action description"
        }
    },
    "objectTypes": [],
    "outputFields": [
        {
            "typeDefinition": {
                "name": "testValue",
                "type": "number",
                "fieldType": null,
                "options": [],
                "optionsUrl": null,
                "referencedObjectType": null,
                "externalOptions": false,
                "externalOptionsReferenceType": null
            }
        }
    ],
    "id": "31903326",
    "revisionId": "1",
    "functions": []
}

Here 'labels.en.outputFieldLabels' is missing but everything else is present