I'll have a look to see if I can find out why it's trying to do that, but it might not be until Monday.

I did a quick test using the bicycle routing you on the OpenStreetMap website (right-click and select route to/from here), from the Green Man to Maryland Point. GraphHopper and Valhalla use the road as expected. OSRM does the same, apart from an unexpected detour around the back of Harrow Green.