Garmin Sensor 2 detects the wrong sensor?

My Garmin speed and cadence sensor 2 were detected by my Mac book air perfectly. It reads the Kph and Rpm on the pair menu screen perfectly. But once in the game. The speed sensor’s data is wrong. I noticed, every time I stopped peddling the speed goes to zero together with the cadence and when I pedal back rpm goes up together with the speed but does not give the correct data on the speed sensor. Does it mean that the cadence and speed were read by the cadence sensor alone? Any suggestion on how to fix this?