I had this exact same problem too and it's been driving me crazy for the longest time!
Here is the fix that I found:
Under the touch screen pull down for <Settings>,< PC Settings>= <PC and Devices> (after you go and click on it - smh) - Must Switch to Modern UI to find this, then search for <PC Settings>
(BTW I hate this Touchscreen Modern UI Menu - I don't ever use it except when I have to, normally I always use the old windows desktop and start menu searches)
At the top of the screen Select "Adjust my Screen Brightness Automatically" switch to OFF!