How did you enable ECG feature, it is an easy configuration?
1. Setup your watch with iPhone
2. Make an un encrypted backup of iPhone from Laptop
3. Then there is a file inside that backup that you have to edit. Very small change.
4. Then restore that backup to iPhone
5. Restart both watch and iPhone and its done

the change you make will be synced to Apple's server and then you will have no issue at all
I did this in 2020 on Series 4 44MM. Still working fine. I am always updating the watchOS and iOS both. Had no issues at all.
Not sure this method is still working or not.
I will try to send you the guide I followed. Or try googling and you will find it.