The New Big Sur OS has an Algorithm built into the OS to monitor the Battery Usage. It is semi-well known that Apple is attempting to Extend the life of Batteries and Reduce the Full Battery Cycles. In so doing - it is normal for the Charger to be attached and the computer will charge to about 80% and Pause. If you check the Battery icon on Desktop - one will see this Pause to Charge. The User can over-ride this , by the same icon , and un-Pause the charging process. Though, if one looks closely - it will probably Pause again at 90%. User can again over-ride by same method until it charges to 100%.
The side affect of above will increase the number of Full Battery Cycles and in effect less the life of the battery.
Personally - on my MBA M1 and MBA Intel 2018 Retina - I leave Big Sur to decide when to fully charge and when to Pause. That is personal Preference and not for all.