This is a case of missing information in OpenStreetMap, cycle.travel’s source data – the road isn’t marked as unpaved. You can fix it by going to OSM, zooming in on the area, clicking the ‘Edit’ button and marking the road as unpaved – there’s a little tutorial which shows you how it works. cycle.travel takes an update from OSM every month or so, so once that’s happened, the routing and mapping here will reflect your changes!