iPhone iOS upgrades caused issue with Apple Watch Series 7

I upgraded my iOS on the iPhone few times since May. Every time after an upgrade, the Cardio Fitness on the iWatch (Series 7) no longer works and I’ve to unpair and pair it back then wait for calibration for few days. However, Cardio Recovery and all related trackings are working fine except for Cardio Fitness. I went to a local Apple Shop and it seemed this issue only happened on iWatch Series 7 recently on iPhone iOS upgrades. Anybody got similar problem and know how to fix it?

[Re-Titled by Moderator]