From my experience you can design and load a route, set way points check it all with Google maps and once on your trip if you pull over for gas or food and it (GPS) says "recalculate" then it will sometimes go brain dead and calculate a new route that will ignore your preset directions.
The way around that is to cancel your trip (on the GPS) and if you are traveling by a stored route just reload but do not navigate to the start point. It will take you to the next waypoint on the trip and back on the programed route.