Map routing uses the underlying data from OSM (https://www.openstreetmap.org) and Mapbox (https://www.mapbox.com/) so if the trail isn't marked properly in their data set Strava routing won't recognize it as a valid way. To work around it, you can enable "Manual Mode" which will let you click to draw lines wherever you want. Once you've connected back to the trail network you can disable manual mode to have it automatically follow roads/trails again.

Both OSM and Mapbox accept corrections and updates, so if you want you can submit changes to those platforms and have the data eventually show up in Strava. Click the little information icon in the bottom right of the map and there's a link to "Improve this map." For OSM you can visit their website and edit data directly. I don't know how often Strava updates/syncs to the latest data so it may take some time for the change to be available.