PATCH /conversations/conversations/{id}/participants not updating custom attributes

Trying to update a participant’s custom attributes mid-conversation using the Genesys Cloud API. Sending a PATCH to /api/v2/conversations/conversations/{id}/participants/{participantId} with the updated attributes object. Getting a 200 OK response, but the changes don’t reflect in the widget or subsequent API calls. Is there a specific field I need to include to trigger the update? Here’s the payload:

{
 "attributes": {
 "tier": "premium"
 }
}

Checked the docs, but they’re vague on mid-session updates.