The first trailer crash wasn't just that it looked like a sign to the vision system. It's that the radar saw it and alerted, but the AEB was configured to do nothing unless both the radar and the vision system alerted. Since it was a white trailer against a bright sky, the vision system couldn't see it.

Tesla later pushed a software fix for the AEB to trigger even if only the radar alerted.

https://electrek.co/2016/07/01/understanding-fatal-tesla-acc...