Phantom braking seems to be caused by either incorrect speed limits in the onboard speed limit database, incorrect lane detection (especially in areas where roads have recently been changed) or incorrect detection of an object in front of the vehicle (typically a rapid light-to-dark transition in the roadbed ahead).
The speed limit issue is primarily due to infrequent updates to the speed limit database - if Tesla is only updating this information every year or two - it won't pick up recent changes, especially in areas that have current or recent construction.
The lane detection is an issue when current or recent detection has caused lanes to be moved, added or removed - when the software believes you are located on an adjacent roadway or ramp, when you're actually in the mainlanes of a highway. Since the software is using the onboard navigation maps, and those have been updated every year or two, this is especially a problem in areas with recent highway construction.
The false detection of objects in the roadbed ahead is due to the onboard software not properly classifying the object ahead. The software is improving over time, though for safety, it's better for the software to slow down if it's not confident about classifying the dark area ahead as a shadow/roadbed color or a real vehicle. This has been improving over time - but understandable that Tesla wants to be more conservative with braking in these circumstances.
The issues with the onboard speed limit and navigation map databases is a significant flaw in the current software design. Longer term, the correct solution is for the software to act more like a human driver, and react to the actual road conditions, reading all traffic/speed limit signs, and detecting the lanes based on the visual input. Until then, unless Tesla shifts to providing up-to-date speed limit and navigation map data to all vehicles, we'll likely see this type of phantom braking.
Due to frequent slowdowns on area roads with current/recent construction, it's so bad, that it may be safer to drive without TACC/AP than to risk rapid braking in the middle of high speed highway traffic - especially when the current software is doing this frequently.
Wish there was a setting to disable automatic speed adjustments... At least until this is more reliable...