There is a known issue with Apple's iOS 14 & 14.0.1 update where your Apple Watch is dropping health sensor readings from the automatic Health synch to iPhone. It will appear as though you were not wearing the Watch for extended periods of time when you actually were. If affects all Health & Fitness apps that read Health data from iPhone. 

Other symptoms include the iPhone heating up, reduced battery life, periods where the iPhone does not respond to touch, missing workouts, lost GPS history, missing noise data and more. 

Apple has provided an official support page with instructions on how to resolve this issue. 

You can view the page here.