I know the issue too and I think it's caused by a too optimistic time calculation for turns plus the vehicle settings ("oh, we didn't use minor roads often enough to reach the desired percentage!")
As we had it a couple of times, default settings are not really usable for daily life. It is way to fast especially in cities. And it is very tedious to move the slider to a specific speed. May be this is because of my small screen (Xperia Xcompact) in relation to my thick fingers :D
For me, it would be much more comfortable, to have an option to set speeds for calculation with a numeric pad.
Possible reason for the detour: For MFN the S 172 (Zellescher Weg) is urban and the detour (Reichenbachstr.) is extra-urban. (In OSM both are tagged with urban.)
Some weeks ago, I was on my way from Chemnitz to Dresden, via the Autobahn A4. Since there was a traffic jam on the Autobahn (checked via Google Maps) some kilometers before Dresden, I wanted to leave the Autobahn early and therefore deselected Autobahn on the routing profile. The remaining distance was some 40 km. Look, what Navigator routed then:
Absolutely crazy, nothing less like this. The first exit, actually some 10 km away, Navigator found, was North of Berlin, but on the "wrong" side of the Autobahn. So Navigator routed further North, turned back on an Autobahn intersection and left the Autobahn somewhere in the wild North of Berlin and routed back to Dresden on non-Autobahn roads. It it obvious, that I did not follow that. I selected then to use the Autobahn, and the routing became normal again, some 40 km left ...
Put the start on Autobahn A4 direction Dresden between exits Berbersdorf and Siebenlehn. Put target at Dresden South for example at Dohnaer Platz. Deselct Autobahn in routing profile. Start simulation. Have fun!
If the map of the Czech Republic is installed, I get the following routes (the first one if "Small Local Road" is disabled, the second one if "Small Local Road" is enabled).
It is even much much more crazy. Start just before Siebenlehn exit on Autobahn A4. Target just behind on Bundesstraße B 101, some 3,5 km. Navigator does not use exit Siebenlehn, if you deselect Autobahn. It goes more than 800 km.
This one tested after a map update on my tablet.
If you are on Autobahn and deselect Autobahn in route profile, Navigator should simply leave the Autobahn at the next exit and stay on non Autobahn roads from there on.
( post above: first 0 should be the little thing means degrees)
Even when I let MF calculate it on the PC it creates a route on the wrong side of the highway. Problem in OSM??? Could no see anything is wrong. Tryed some other points on our city-ring. When its shorter to drive on the wrong side, it will be calculated.
A 14 in Germany just north of Dreieck Nossen direction Leipzig. Target was somewhere in Dresden. Switched off Autobahn in routing profile while navigating:
Didn't follow this, just left the Autobahn at the (next) exit Nossen. Navigator automatically rerouted, made just 37 km from 800+.