Issue reported by Paul Toigo here:
It's been a while since I imported a route, but when I did so with this 411.3km RWGPS route:
https://ridewithgps.com/routes/10394478
be it TCX or GPX, Plotaroute claims the course is 318.523km. I tried exporting/importing from Plotaroute to no avail, but when I first import to Strava (reporting a 410.78km length), export a TCX, and read that into Plotaroute the length is a tolerable 404.801km.
Bug reported to the dev team.
Note: plotting the same route by hand results in a close enough distance of 408.834 km, so the upload feature seems to be the issue.