I am having the exact same issue. I have just purchased cheap Magene cadence/speed sensors from Aliexpress and had my first two rides this morning and I can confirm that while the cadence sensor works as expected, data from the speed sensor does not appear in Health app and I believe that during the workout the speed that was showing up was indeed from the GPS (I had a typical delay in speed decreasing over long period of time after quickly stopping on red lights). I will try to swap these sensors next time I am going for a ride to make sure the sensor is working correctly, but it looks like watchOS 10 bug. EDIT: I have submitted a feedback with ID FB13046304

Yeah I would love to learn that too, since some of my existing complication images look too small for the new 45mm apple watch