Web Messaging Guest API: Typing indicator POST ignored

The docs say POST /api/v2/conversations/messaging/{id}/typing should work for guest sessions, but it returns 403 Forbidden. We’re using the guest token from the handshake response. Here’s the payload we’re sending:

{
 "from": {"id": "guest-123"},
 "to": [{"id": "agent-456"}],
 "type": "typing"
}

Is this endpoint actually restricted to authenticated users only?