PATCH /api/v2/conversations/participants attribute update failing with 400

PATCH /api/v2/conversations/participants/{participantId}

400 Bad Request

Trying to update custom attributes on a live chat participant. The payload looks correct, but the API keeps rejecting it. I’m passing the JSON body directly.

{
 "attributes": {
 "wem_queue_priority": "high"
 }
}

The documentation says this endpoint supports attribute updates, but I’m getting a generic validation error. Am I missing a required field or is this endpoint read-only for attributes?