Screen turns dark when the remaining battery is less than 5%. Why?
When the remaining battery is less than 5%, battery consumption is fast due to settings such as screen brightness and CPU.
When the remaining battery is less than 5%, the phone will adjust screen brightness to 0% to restrict use of unnecessary features
to improve the battery efficiency.
How to fix
Please be assured that it is a sign of normal operation.
This feature has been offered for Optimus G Pro and later models.
(Models from other brands also provides a similar feature)