Data Action 400 Error Mapping Zendesk Comments

Can anyone explain why the custom Data Action fails with a 400 Bad Request when mapping Zendesk ticket comments to Genesys Cloud interactions? The payload structure matches the Zendesk API response exactly, but GC rejects it during the migration sync.

{
 "type": "comment",
 "body": "Customer inquiry regarding order #123",
 "created_at": "2023-10-27T14:30:00Z",
 "author_id": 98765
}

This works perfectly in Zendesk, yet the GC Data Action endpoint throws a validation error on the author_id field.