POST /api/v2/interactions/participants/{id}/attributes 400 on live voice call

Pushing custom attributes to a live voice participant via the DFO API is throwing a 400 Bad Request. The interaction is active, and I’ve verified the participant ID from the initial POST response.

The payload is minimal:

{
“attributes”: {
“ext_sys_ref”: “12345”
}
}

The error message just says “Invalid request payload”. No specific field validation errors. Is this endpoint locked for voice interactions once the media session starts?