EventBridge payload missing conversationId in Genesys Cloud

Why is the conversationId missing from the EventBridge detail object for conversation:participant:created events? The webhook test works fine, but the live stream drops the ID. Here’s the payload I’m getting:

{
 "detail": {
 "eventType": "conversation:participant:created",
 "organizationId": "org-123",
 "userId": "user-456"
 }
}

The API docs say it should be there. Am I misconfiguring the rule filter?