There is something strange in the path calculation that goes wrong when the distance between 2 points is large ~>80 miles (130km) if Offline & Track navigation are enabled
The calculated path can/will deviate from a simple direct path down a motorway to leaving via an off ramp, re-routing around a load of side roads & sometimes even jumping up/down off of elevated roads before continuing

To test this I simply create a 2 point route larger than > 80 miles starting at an on ramp & finishing at an off ramp
The web planner will show me a distance & time for the route
Open the route in the App & the same distance & time are displayed before the navigation is started

When the navigation starts the route is recalculated & it is immediately obvious that something is wrong because the mileage & time increases
Inspecting the new calculated route, incorrect / unexpected deviations off of the motorway are easy to find

Shorten the route so it includes the areas where the unexpected deviation occurred & the path follows the correct direct route along the motorway
It is the large distance/time between points that seems to cause the unexpected deviations

Online & disabling track navigation the problem does not appear
It has nothing to do with traffic or road works

I don’t really care / am not expecting a fix for this as I rarely plan trips with large sections of motorway,
it is just something I noticed when testing to try & answer this other question on the forum
Just thought maybe as it is a very reproducible error it may give a clue as to where in the path calculation sometimes an error can occur