Data with current layout option for custom visual using matrix dataViewMapping

[smpa01]

I have a custom visual that uses a matrix dataViewMapping (rows + values, no column grouping). When I upload the .pbiviz to the Power BI service and use Export data from the three-dots menu, the "Data with current layout" option is disabled.

"This option is only available for table and matrix visuals"

My visual renders a plain HTML table from the matrix dataView - it is a matrix visual. But the option is greyed out

{
  "dataRoles": [
    { "name": "rows",   "kind": "Grouping", "displayName": "Row Groups" },
    { "name": "values", "kind": "Measure",  "displayName": "Values" }
  ],
  "dataViewMappings": [
    {
      "matrix": {
        "rows":   { "for": { "in": "rows" }, "dataReductionAlgorithm": { "top": { "count": 1000 } } },
        "values": { "for": { "in": "values" } }
      }
    }
  ],
  "privileges": []
}

Question: Is "Data with current layout" only available for PBI's own native Table/Matrix visuals, and never for custom visuals - regardless of which dataViewMapping they declare? If so, is there a capability flag or API to enable it for custom visuals?

@dm-p 
Thank you in advance.