I had the same problem and fixed it like this:
free and new solution in page 2 FREE Solution 1. Download and install Belle Extra Buttons (Trial) by MrAlshahawy, launch it and set brightness to 100%, and unistall this app. Now you shoud have 100% brightness on you phone but there will be new problem, now you can't regulate your brightness, it allways will be 100%
2. Download and install Battery Optimiser by MIGITAL. With this app brightness will be reduced.
PAID Solution:
Buy and install Extra Buttons by LanternSoft and regulate brightness like on a Belle Extra Buttons (Trial) app. check my other post in page 2