Direct link to the NTSB preliminary report is here: https://www.ntsb.gov/investigations/AccidentReports/Pages/HWY19FH008-preliminary-report.aspx
As expected, this crash was virtually identical to the Brown one almost 3 years previous. The NTSB Chairman's comments summing up that accident could be copied verbatim for this one:
“While automation in highway transportation has the potential to save tens of thousands of lives, until that potential is fully realized, people still need to safely drive their vehicles,” said NTSB Chairman Robert L. Sumwalt III. “Smart people around the world are hard at work to automate driving, but systems available to consumers today, like Tesla’s ‘Autopilot’ system, are designed to assist drivers with specific tasks in limited environments. These systems require the driver to pay attention all the time and to be able to take over immediately when something goes wrong. System safeguards, that should have prevented the Tesla’s driver from using the car’s automation system on certain roadways, were lacking and the combined effects of human error and the lack of sufficient system safeguards resulted in a fatal collision that should not have happened,” said Sumwalt.
BTW, the speed limit on the road is 55 yet the Model 3 was doing 68. A/P was supposedly modified after the Brown crash to limit set speed to no more than 5 mph over* the speed limit, so how was it possible to even engage it in this case? Meanwhile, lots of current reports of safety glitches with the most recent version, 2019.12.1.2, on TMC.
*Itself a safety flaw, as one of the ways that AVs will make driving safer is that unlike humans, they'll obey all traffic regulations. Of course, many speed limits are set below the road's design speed, so once AVs have become the majority we'll likely see speed limits raised, or at least adjusted in real-time to account for changing conditions.