I've reviewed the following posts but there is no trace of the submitted form in InvalidSubmissions or RoutingError folders, or anything in Event Viewer around the time of submission that I can identify.
We have one Forms server running 11 Update 3. Confirmed public user is receiving the Thank You page and preview of the form.
Even though the routing service is supposed to retry any stuck submissions we've manually restarted the service but no new forms have been processed. It's not every submission that is missing - seems completely random.