Override iPhone's automatic brightness adjust?
Settings app will help you find this if you search for Auto-Brightness 



The Auto-Brightness switch was moved in iOS 13.

the Source of the images below is this article:

Go to Settings -&gt; Accessibility -&gt; Display &amp; Text Size.  Scroll down to the bottom of the page and you will see the Auto-Brightness switch.

From here you can turn off this feature and, hopefully, this battle with iOS will be won!