If you want the watch to charge completely up to 100%, you will have to disable the Optimized Charging Limit which will then charge your watch up to 80% as a first step, and then up to 100% if the usage pattern signals that you need the watch to be at 100% at a certain point.
Otherwise the battery will only be charged up to 80%
My iPhone charges up to 80% during the night and up to 100% just before I usually get up.
As I wear the watch during the night, I charge it if needed before I go to bed up to 100%, so the watch gets never charged up to 80% automatically, although this option is enabled.
My Apple Watch Ultra 2 uses more battery life if I enable the Live Activities during workouts in the settings for the Smart Stack. Yesterday I lost approx. 3% battery, during my 30min Cycling Outdoors workout.
Today, without Live Activities enabled during the day and 30min workout cycling, the usage is back to normal, 18hours after a full charge, 78% battery left, 1.2percent/hour.