I’m having the same issue with my Watch (45mm, Cellular).
Even the update to watchOS 8.1.1 didn’t fix it.
It’s just annoying.
The first days the Watch charged fine and ultra fast (15% in 10min or so), but now I’m just having problems.
It’s not fast charging anymore. Ist not charging normal anymore. It’s some kind of an ultra slow charging mode, some kind of 10% in 30 min or even less. Sometimes the night mode is constantly turning on and off or the lockscreen apears.
Only thing that helps is to hard reset the Watch. But for now my new Watch is just a disappointment. Just unreliable charging. Never had such issues with my series 3.