By chance I came across a feature, but I'm wondering, how it really works: Planning along GAFOR routes in Austria

Planning a flight from LOWL to EDMV with "LO17" in the "route" field, leads to a nice flight plan along GAFOR route LO17, with several waypoints:

LSZH to LSZG via S12 works fine as well

BUT

Trying the same from LSZR to LSZH via "S13" brings up a straight magenta line and the following message, although the given airports are the official start/endpoints of S13

What's the logic behind this feature and how can I use it properly?