Conversations API PATCH for participant attributes returning 422 Unprocessable Entity

Trying to update participant attributes mid-call using the Conversations API. Sending a PATCH to /api/v2/conversations/voice/conversations/{id} with a JSON payload containing the new attributes. The request keeps failing with a 422 Unprocessable Entity error. The error message just says ‘Validation Failed’ without specific field details. I’ve checked the token and the conversation ID is valid. Has anyone seen this before? Here’s the payload structure I’m using:

{
 "attributes": {
 "customFlag": "true"
 }
}