@dipayanbiswas what you have here, is a backlight strip that is failing (left hand side darker than right due to failing LED's). When you decrease the brightness you decrease the power, that allows the failing section to turn back on. The only fix would be repairing/replacing the backlight or continue to run the display at limited brightness.
I believe that for your models display, the LED backlight is part of the assembly. You would most likely have to replace the screen. Enjoy using it until it one day will stay black. Then you know what you will need to do.