There was recent update that affected a few top end models.
Start basic diagnostics by force restarting the Watch and paired iPhone at the same time:-
Force restart the Watch
i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.
Then while Watch is restarting, force Restart the paired iphone Force restart iPhone – Apple Support (UK)
Test it for a suitable period depending on the nature of the problem.
If it is still misbehaving...
Unpair and set up again, takes about 20 minutes.
Unpair and erase your Apple Watch – Apple Support (UK)
Set up your Apple Watch – Apple Support (UK)
If you've completed that troubleshooting and the issue is still not resolved, Contact Support but I dint think you’ll need to.
Click here > Contact - Official Apple Support
or phone numbers
Click here > Contact Apple for support and service - Apple Support
and or Genius Bar Reservation and Apple Support Options - Apple