I have the Edge 830 and I have a Wahoo ANT+ cadence sensor, couldn't get a Garmin one at the time, but the wahoo one works perfectly when riding. But when doing the averaging the Garmin gets it massively wrong. Eg I completed a 100km ride, the average cadence on the 830 is 95, which is not correct, and the Max cadence is 18, which is impossible.. If you look at strava it will calculate these correctly, Garmin Connect just mirrors the 830..
Any ideas?