Small route adjustment changes major sections

https://cycle.travel/map/journey/517820
If I make a small route adjustment, to push off a side-road on to a route after Niort, the website then adjusts a large routing section north of Niort, miles away, pushing off minor road/tracks onto larger roads.  It seems unnecessary, can't be undone, and is easy to miss the change as I've already checked the earlier section of route.  I think this may have also happened on other routes, but it was obvious on this.