hi,
I'm having the exact issue that started this thread and the 1809 then 1903 update.
To answer your above question and changing the "Display power saving technology" has NOT effected the issues in any way
"your brightness doesn't automatically change between plugged in and battery" - YES this is the issue
"but you can change it manually or you can't change it at all?" - Yes I CAN change it with no issues.