Set Participant Data action dropping custom JSON in IVR flow

How do I use the Set Participant Data action to pass custom variables through an IVR flow? The action returns a 200 OK, yet the event payload strips everything except the default interactionId. I’m sending this JSON structure from the Python backend, but the downstream webhook doesn’t pick up the queue_priority field:

{ "queue_priority": 5, "customer_tier": "gold" }

Just need the exact JSON schema for the participant data object.