I am assuming the package would eventually be passed to Royal Mail for delivery? Royal Mail will have procedures for deliveries to the passport office in place as I assume they deliver thousands of items daily so they wouldn’t turn up early and no one be there, there will be a delivery plan in place.
So maybe the issue is with the French mail notification system not talking to the the Royal Mail notification system – chances are the item is there, but the notifications have just not worked across the two systems – however the only way you’ll find out if this is the case is to keep an eye on the application for a change in status.