A shorted iPhone 6 backlight is usually caused by poor installation of the new display. Either the battery was not disconnected when the LCD FPC was making contact to the board or the solder joints for the backlight of the display are exposed and can be shorted when making contact with metal.
This filter is FL2024 and you can usually tell if it is shorted by a brown discoloration on the chip. Or you can use a multimeter to test for continuity.
This is a repair we do often and usually completed the same day.