Got a rather perfunctory response from Tesla Executive Care:
Thank you for contacting Tesla! I apologize for the concern you are having regarding your TACC and autosteer. Your local service center would be best suited to assist you in this situation as they can ensure your vehicle is working properly and if need be go on a road test as well. I have provided their contact information below:...
I was hoping for something better than this. It's obvious that the problem is either intermittent or only occurs under very special circumstances. In that light I provided as much information (time of day, location, direction of travel, etc) as I could and even granted permission (if that's even needed) to access the car logs to see what factors influenced the sudden braking.
This is a serious problem both from the standpoint of confidence in the safe operation of the AP system and in avoiding an accident where the car is rear-ended because the car behind is not able to react to such a sudden and unexpected braking action.
Anyone have any ideas how to pursue this further?