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 the restart.
It didn’t resolve the issue.
I go to bed last night with it fully charged and woke to my Ultra at 50%.
The kicker is its discharging at a faster rate than it charges.
I’ve resorted to resurrecting my Series 5 until Apple puts out a fix for this.