Try and update your watch and iPhone to the latest available software to see if the problem persists
First, check this:
1. in the watch app on your iPhone, go to my watch>privacy>check that fitness tracking and heart rate are enabled
- Go to my watch, workout, turn off power saving mode
- In the watch app, go to my watch>passcode>turn wrist detection on
After that try and restart both your iPhone and watch, while restarting your iPhone first
Are you using third party apps for fitness tracking, sleep tracking or heart rate, you might want to see if they are active, and if so, remove them temporarlily to see if the problem is caused by this apps
If the issue persists ,Contact - Official Apple Support to set your device up for service