iOS 13 comes with many new features and tweaks. As far as I can tell, it comes with the Auto-Brightness feature activated. Auto-Brightness adjusts the brightness of your iOS Device screen based on the surrounding light. If you are in bright light, Auto-Brightness cranks up the screen brightness. If you are in a dark lighting situation, Auto-Brightness turns down the light on your iOS Device. If you dislike this feature, here is how to turn it off.
I have the Auto-Brightness feature turned on, I think it helps me in varying lighting situations. However, if you would like to turn it off, then head on over to Settings, scroll down and tap on Accessibility:
Once you are inside the Accessibility area tap on Display & Text Size:
Then, scroll down to the bottom of the window to find the Auto-Brightness setting:
All you have to do is tap on the green slider and the Auto-Brightness feature will be disabled. Keep in mind you will have to adjust the screen brightness of your iOS Device manually.
I know some people do not like the Auto-Brightness feature and prefer to adjust their iOS brightness manually. I used to do this, but prefer the Auto-Brightness feature, at least for now. If you wish to disable this feature, you know where to go.