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).
I tried a few of the suggestions offered in this thread and the only real solution that helped me with my issue was trying out the public beta watchOS 10.2 update. This is also after restarting and unpairing the watch from my phone.