The curve side down is the original GM position. But as you mention, the sensor is indeed then mostly blocked. So it doesn't work too well. There were apparently multiple dealers (including mine) flipping it around, curve side up, sensor down, so that it DID see lights from the back, and work better for auto dimming. But then it didn't provide the full view out the rear for the driver it was meant for. Attached is a picture from the Chevrolet website in 2014. My Volt is a Dec 2013 build. I received my 2014, new from the dealer, sensor down, curve side up. I flipped it around later that year. Been that way ever since.
