Web Messaging Guest API: Typing indicators failing with 400

Sending a typing indicator via POST /api/v2/conversations/webmessaging/messages returns a 400 Bad Request. The payload matches the schema docs exactly:

{
 "to": { "id": "conv-123" },
 "type": "text",
 "text": { "content": "" }
}

Removing the text object works for regular messages but breaks the typing state. Anyone else hit this wall?