I updated to the public release (9/12/2022) of iOS 16 and it fixed the issue. My theory is that after downgrading from the beta of iOS 16 the Fitness and Health apps also updated and retained data which was from a newer OS (ios 16) and after downgrading and restoring backup it would have caused the issues. So if you have an iPhone 8 or later (mine is a 12 Pro) just upgrade to the full iOS 16 and problem is solved. (15.7 did not solve the issue).

Hi. I have the EXACT same issues. I have an iPhone 12 Pro, I upgraded to IOS 16 Beta but had issues with banking app. I went back to IOS 15.6.1 but Fitness on the iPhone just gives a black screen with the spinning gear and Health cannot show my data. Also when I try and enter health details on the Watch App and click Done it does NOT save.