slow start up stores settings like this BUT that wont last. it will go away.
The issue has NOT been fixed and there seems to belittle hope that it will be fixed.
Sorry, what is "slow start up"? Brightness changes and this means that some portion of programming code inside Windows determines if power is plugged in and adjusts light according to the settings. I think if MS will not issue special fix to remove this code, it will work further. And MS will definitly not issue a fix to remove smth they do not care at all.