Had you already ridden that route previously? Zwift only displays the ‘End of route’ achievement the first time you complete it, so if it was your second time then you wouldn’t see anything.
Route completion is something that Zwift could deal with better and it’s been asked for: https://forums.zwift.com/t/end-of-route-congratulations-or-at-least-an-indicator/15266
Also, some routes have a section of ‘lead-in’ to ride before you actually start the route properly. Zwift Insider has a good list of routes and their details: https://zwiftinsider.com/route-details/