I am on Catalina with two MacBook Pros, but starting with Catalina Apple modified the charging process to prevent the battery from staying at 100% for long periods. The battery (under Catalina) slowly discharges, once every day or two, with the charger connected, until it reaches ~ 90%, then it charges back up. I believe you can force it to charge immediately by rebooting, but after I understood that this is the desired battery management process by Apple, intended to prolong battery life, I started to ignore it.
Under Big Sur the details may differ but I believe you will see the same thing, essentially. I believe Big Sur may hold the charge at something less than 90% for some time, but then recharge back up. If your battery NEVER recharges back up to 100%, then something may be amiss with your charger or your battery.
Another "feature" is that the max level of charge (in maH - milliamp-hours) will slowly be reduced as the battery ages. It may show 100% charged but it will be to a level lower than when the battery was brand new. This is also intended to prolong the life of the battery. The downside is that the highest battery level may be slowly reduced as the battery ages, but the intention is that it will last longer than if one keeps it pegged at full charge and 100% all the time.