I’ve noticed this with my Fitbit. I find it annoying how it converts everything to steps when riding a bike (motor and pedal). Having said that my heart rate does goes into the “fat burn” zone whenever I’m on a motorbike ride and if it’s a track day It will go as high as “cardio”. So even though your sat on your arse your still getting a workout. I,d love to see the HR of a rider doing a lap at the TT.