Open Messaging API: Quick reply buttons not rendering in Web Messaging SDK

I’m trying to send a card with quick reply buttons using the Open Messaging API, but the SDK just shows the text instead of the interactive buttons. Here is the JSON payload I’m posting to /api/v2/conversations/messages:

{
 "type": "openMessaging",
 "omMessage": {
 "version": "1.0",
 "payload": {
 "type": "text",
 "contentType": "text/plain",
 "content": "Select an option:",
 "elements": [
 {
 "type": "button",
 "title": "Option A",
 "action": {
 "type": "postback",
 "payload": "option_a_selected"
 }
 }
 ]
 }
 }
}

The API returns a 200 OK, but the widget renders it as plain text. Am I missing a specific flag in the omMessage structure?