After I updated my Series 8 to watchOS 10.1 I had terrible battery drain. Something like losing 10% in 20 minutes.
I tried to force a restart (if you’ve never done hold the Digital Crown and the side button together until the Apple Logo pops up).
After that things got back to normal. Lost about 11% with an hour of GPS workout, which would equate to a bit more than the 7 hours of GPS workouts that Apple states (and I’m on 98% battery health).
After the 10.01 update my series 7 battery would go from full charge to lass than 10% within 3 hours. I tried the hard restart, put my watch back on the charger and the same thing would happen. Finally I unpaired the watch and paired it back and the problem has been fixed.