PATCH /api/v2/interactions/{id}/participants returns 400

NICE CXone API & Integrations
1

Trying to update a custom attribute on an active voice interaction via the Conversations API. The request hits PATCH /api/v2/interactions/{interactionId}/participants with a valid token and correct division ID.

PATCH /api/v2/interactions/{interactionId}/participants
{
 "attributes": {
 "crm_ticket_id": "12345"
 }
}

The response is a 400 Bad Request with no useful error message in the body. The attribute exists in the schema. Is there a specific flag needed for mid-call updates?