Is this route by any chance marked as an offroad route in the online routeplanner? @Herman-Veldhuizen
Offroad routes are not (yet) supported and can indeed give weird results! I am investigating the possibility to support this 🙂

@M-Schrijver I have had similar problems on my BMW sat nav. The thing they have in common is the Here maps. I've also had Here maps try to take me via Belgium to go 10 miles down a road in Spain and onto a Toll road because the Here map didn't know.