After my Autopilot 1.0 test drive experience with a Tesla sales person, I am not surprised at the instructions you were given. Mine had me engage AP in stop and go traffic with TACC set at 65 and a large gap to the car in front. That caused the car to accelerate towards the stopped car in front of me, and every time I hit the brakes to stop the car (because I dont like accelerating towards stopped cars) the sales person told me to not brake. I guess I was supposed to trust the car. (Which probably would have been fine, except when its not, as in your case) I came away perfectly happy to order a AP 0 (no AP) CPO after that.