Open Messaging API drops card payload on POST /conversations/messaging/messages

Pushing a quick reply card via POST /api/v2/conversations/messaging/messages returns a 200 OK, but the guest only receives the text fallback. The trace spans show no errors, yet the content block with type: "card" vanishes from the delivery log.

{
 "content": {
 "type": "card",
 "text": "Select option",
 "actions": [{"type": "quickReply", "text": "Yes"}]
 }
}

Am I missing a specific header for structured content?