Brightness getting stuck never happened on my 950XL even when I opened its box. If it happened to me I could have heart attack

I suspect you have a combination of some updates not installed properly AND buggy apps, or even if you have a SD card and moved apps from device to SD or vice versa. such combo problems best solved by a clean hard RESET of phone AND not using back up copy but rather reinstalling each app from scratch.
I am also finding frequently I have to uninstall then reinstall some apps that then run smoothly plus solve some other problems that I never could fix before.