Export Reports to File Using SPN Access?

So I followed the advice of adding the identity specs to my export request, as was described here --> https://community.powerbi.com/t5/Developer/Rest-API-Export-Report-to-PDF-that-uses-Azure-Analysis-Service/m-p/1133666#M23967. 

Same result (see below). This service principal has full access granted to the Power BI workspace. So I'm at a loss. 😞

POST https://api.powerbi.com/v1.0/myorg/groups/{group_id}/reports/{report_id}/ExportTo HTTP/1.1
Host: api.powerbi.com
Authorization: Bearer {access_token}
User-Agent: FxVersion/4.700.19.60701 OSName/Windows OSVersion/Microsoft.Windows.6.1.7601.Service.Pack.1 Microsoft.PowerBI.Api.PowerBIClient/3.11.1.20103
Content-Type: application/json; charset=utf-8
Content-Length: 638

{
  "format": "PDF",
  "powerBIReportConfiguration": {
    "settings": {
      "locale": "en-us"
    },
    "pages": [
      {
        "pageName": "ReportSectionbe63eaf77c6775278e11"
      }
    ],
    "identities": [
      {
        "username": "{spn_object_id}",
        "datasets": [
          "{dataset_id}"
        ],
        "roles": [
          "client"
        ]
      }
    ]
  },
  "paginatedReportConfiguration": {
    "formatSettings": {
      "PageHeight": "14in",
      "PageWidth": "8.5in",
      "StartPage": "1",
      "EndPage": "4"
    }
  }
}
HTTP/1.1 401 Unauthorized
Content-Length: 153
Content-Type: application/json; charset=utf-8
Strict-Transport-Security: max-age=31536000; includeSubDomains
X-Frame-Options: deny
X-Content-Type-Options: nosniff
Access-Control-Expose-Headers: RequestId
request-redirected: true
home-cluster-uri: https://wabi-us-central-a-primary-redirect.analysis.windows.net/
RequestId: 6937aa84-e2e1-4cda-a7fe-53c9e28f52a8
Date: Thu, 11 Jun 2020 17:56:19 GMT

{"error":{"code":"PowerBINotAuthorizedException","pbi.error":{"code":"PowerBINotAuthorizedException","parameters":{},"details":[],"exceptionCulprit":1}}}