Had mine only 2 weeks, but have had this happen once while exiting the freeway on NOAP. The car gave a few sudden brakes with nothing in front of me but plenty of road. Not sure why. I suspect the computer errs on the side of paranoid braking rather than ignoring a signal that could be noise. Until the AP computer is able to improve signal to noise, this is probably the preferred behavior.
When taking freeway exits NoA may indeed slow down intentionally. You can see the Max Speed indicator on the display actually change be 5 MPH at a time. If it's a transition to another freeway, the speed should automatically return to your setting when you have merged onto the new freeway. If it's an exit to city streets NoA will slow down quite a bit and turn itself off, leaving just standard AP.