my opinion --
assuming you have the new ultra 2 black? thats what i have. its just been a week since these have been out. if you put the watch on charge at different times and you use it more on one day than the other, it will take some time for the watch BMS to get a good handle on your charge times. its probably learning -- you just need to give it time. i just charge it every 3rd day to some random level ( 80% 75% etc..) and go on with my day.
if you dont have the newer awu 2 black and you just activated the optimized charging, the same thing applies -- you need to give it time. when i get a new mbp, i always use charge to 80% . it will take it DAYS to figure out my schedule because i carry my laptop to the office and home daily. eventually it figured my monkey ass out and now it charges to 80, sometimes 100 ( normal).