The Montana is "suppose" to take your Route from BaseCamp (map versions being equivalent) and run them unchanged. But, I have come across Routes that would lock up the navigation at certain places along the route. I believe that these lockups are caused by Via Points placed at Intersections. This does not happen all of the time - which makes it very hard to diagnose just what the heck is going on. I have never had a Route lockup if I placed my Via Points after Intersections when I needed them.
As has been discussed, if you want to make sure your route will calculate properly create a Route in BaseCamp that follows the path you want and then create a Track of the Route. Just before sending your Routes/Tracks to the Montana, change your Routes to Direct Routes. Once on the Montana make sure your Routing Activity is set to Automotive or Motorcycle Driving. This will cause the Montana to calculate your Route into an along-road route. Now you can compare it to the Track as mentioned and verify that the Montana calculated the route as you intended. The side benefit of this process is that you can now EDIT this route on the Montana - including reversing the Route.