I have a gravel bike with four wheel sets. Only one wheel set has the speed sensor. My garmin 1030+ switches to gps mode when I use the wheels without the sensor.
I know it's using the speed sensor because I check it before each ride. I pickup the bike and pedal in place ( bike not moving / rear wheel spinning ) and the head unit shows speed is moving.
I have had my heart rate monitor or maybe it's the head unit freeze at one heart rate say like 119 even though I know I am more like 150+ while going up hill. I think that's because of low battery though in the heart rate monitor.