With a MacBook Pro 16-inch, 2019 running Monterey 12.1, Optimized Battery Charging has recently stopped keeping the battery charged at about 80 percent as it had previously been doing.
Instead, with the Power Adapter plugged in, the battery is charged to 100 percent and stays there.
Mine did this after I used it on battery power one day. For a few days it stayed at 100%, and then it went back to the old 80% behavior.
My conclusion is that if you use it on battery power occasionally it thinks that means it doesn’t need to do the 80% thing, but once it figures out you primarily use it plugged in, it reverts to optimized. So give it a few days and see if it goes back.
My MacBook Air (M1, 2020) running Big Sur 11.6.2 behaves as the OP described. It has been several days since the battery level dropped as low as 98%, but in the past, it would routinely charge only to 80% even after some minutes of battery use. In other words, I used to drain the battery from 80% to (say) 78%, and it would recharge to 80% and show some message like Rarely used on battery.
However, about 17 days ago, I did use it extensively on battery, with the level dropping to maybe 40%. And most days, I use it on battery power for a few minutes at a time (with perhaps a battery level drop as low as 98%). Maybe it’s still guessing what I’ll do in the future and trying to be prepared.
And most days, I use it on battery power for a few minutes at a time (with perhaps a battery level drop as low as 98%). Maybe it’s still guessing what I’ll do in the future and trying to be prepared.
Or it might need to dip below 80% again. That’s what happened to me: as long as I kept it above 80% it kept charging to 100%. When I used it more one day and the level dropped down below 80%, it started charging only up to 80% again. As far as I can tell, only stopped charging if it hit 80% while charging. If it was already over 80% when I plugged it in, it just kept charging to 100%.