Phantom breaking is a real phenomenon while on AutoPilot, where the car decides it needs to stop quickly for no human-obvious reason. There are many theories as to why (overpass shadows, perhaps bad GPS reading combined with inaccurate speed limit data), but nothing has really proven to be the absolute and all-encompassing answer. If phantom braking happens, it does tend to happen at the same place and time, and it can be scary. This is why it’s important to pay attention at all times.
Instances seem to reduce on each software update, and most people never experience it, but it cannot be totally discounted as resolved.