Watch won't charge beyond 80% after OS11 upgrade
Installed WatchOS 11 yesterday on my Series 9 watch. Despite being on its charger overnight, it won't accept a charge beyond 80%. What happened?
Apple Watch Series 9, watchOS 11
Any chance, you have Optimised Charging and/or Charge Limit enabled?
Have a look at the related article:
About Optimized Battery Charging on your Apple Watch - Apple Support