Strange situation. Overnight, my Air M1 used to charge to 100% overnight, with a pause for a while. Since I've upgraded to Tahoe, it only charges to 80% overnight. It does manually charge to 100% though. Can anyone shed any light on this?
You may have Optimised Battery Charing switched on (which is on by default). It can take a while to learn how you use your Mac.
To find this setting click System Settings > Battery > Battery Health (Click the little I button) > then you should see at the Bottom "Optimised Battery Charing".
I personally would leave it on, but if you disable it you should find your Mac will charge to 100%