I have autopilot on my MY set to drive 4 mph over the speed limit. It usually works fine, but recently on the highway the speed limit dropped from 70 to 60 and the car kept going at 74 MPH. The screen displayed the 60 mph speed limit sign, and when I double tapped it the speed adjusted down to 64, so I know the car was aware of the speed limit change. Any idea what's going on?