By now you've probably realized that this behavior is not a defect particular to your car - it is a 'feature' of autopilot, and one of the reasons why autopilot always seems a bit half-backed, even though Tesla has been at it for 5 or 6 years. I use autopilot if I really have a lot of energy and concentration, so I can monitor it and stay on top of its unusual and sometimes unpredictable behaviors. But if I just want a relaxing drive, I turn it off and steer myself.
TACC, on the other had, seems pretty solid. But when you use it, don't forget the Tesla warning that if you are driving over 45 mph, the radar will not see any stationary object (which is why it runs into fire trucks, semi-trucks, and concrete barriers). So if you have no cars in front of you, don't tune out thinking that no traffic means that the the car will handle it - it might not.