braking acc adaptive cruise safety

Scenario: Heading south on 4 lane highway in the left hand lane. Highway generally flat with no obstacles...clear eye view as they say. Traffic volume light, ACC set to speed limit - 45 mph. ACC distance bar is set to one. Burgundy Subaru Forester at 1 o'clock off right front bumper running...

Wondering if others have experienced spontaneous braking while using adaptive cruise. Was in the center lane on the interstate at about 60mph, with no other traffic ahead or behind for at least 1/4 mile, when the car suddenly decelerated briskly for no obvious reason. My wife observed that there...