I did had a similar issue in past, given that I after learned that it is not healthy to charge a lithium battery 100%, it could be or SMC issue so resetting should fix it, or inaccurate software measuring incompatible libraries with the current MacOS, or a faulty battery, or less-powered Magsafe charger.
I don't exclude that patching could alterate battery values (not physical but logical).
Anyway most probably is the software (Battery Health I guess) that needs an update to support current MacOS libraries.
edit:
I thought you were referring to the Amperage (mA) instead of mAh capacity, an inadvertence of mine but I confirm what I have written before.