MacBook backlight turns off when brightness is set at 100%
The issue is caused by a burnt out soldering attaching a cable to an iron bar component situated in the lowest part of the screen. It is part of the module named 612-0035-a. It is the LCD inverter. It is not user solderable with standard equipment.

There's no need to buy an entire screen to fix this, just a new LCD inverter. To prevent same damage from happening again, it is recommended to brace from power surges.