yore Interesting, so it recalibrates during the first charge after every boot.
Not sure where you got this idea, that's not how it works. I was attempting to explain the expected behaviour of the Charging optimization feature which is designed to stop charging at 80% + Shield and switch to charging bypass (allowing the phone to be powered by the USB-C port).
I guess I wasn't clear enough so I'll reference the OP at the start of this thread.
GrapheneOS the device will occasionally need to charge to 100% in order to recalibrate estimated battery capacity.
GrapheneOS Once it reaches 100%, it needs to be allowed to stay there for a bit to truly reach full battery charge. The shield icon showing charging bypass is active will appear. After the shield appears, it will go back to not charging the battery above 80% again. Since it has charging bypass, it won't start dropping from 100% much until you unplug it since it's directly powered from the charging cable as usual.
As Oggyo said it is an upstream bug, some are experiencing it, other's are not.
In my experience I've found that when I run the battery dry numerous days in a row, it takes a couple charges to 100% before it full recalibrates and stops at 80% on the next charge. :)