Guest API 401 on POST /api/v2/conversations/messaging/messages despite valid auth

Need some help troubleshooting

POST /api/v2/conversations/messaging/messages
Authorization: Bearer <valid_token>
Content-Type: application/json

{
 "from": { "id": "guest-uuid" },
 "to": { "id": "routing-profile-id" },
 "text": "test"
}

Returns 401 Unauthorized. Token works for analytics endpoints. Why is the Guest API rejecting the auth header?