I think this is a download error
See https://www.plotaroute.com/route/500283 . This is an absent area
I'm afraid we're never going to be able to get 100% coverage with the elevation data, so there will be occasional gaps in coverage like this, particularly in more remote regions. I don't think there is a viable solution for us to fill these gaps at present.
There are obvious holes in the coverage
For example https://www.plotaroute.com/routeprofile/479615
Planning a route through Sweden at the moment and this is a huge help.
We're pleased to advise that we've extended the coverage of our elevation data to include parts of the far north of the Northern Hemisphere, which were previously not covered. This means you can now get elevation charts and statistics for routes anywhere in Sweden, Finland, Norway, Alaska, Russia and Canada.
You can see provisional elevation profiles for your routes as you plot them in the route planner but more detailed charts and statistics are available in the Route Profile tool after saving your route (see example route profile). You can also print, download and share your route profile from here.