This happens when it is only lighting up half of the backlight LEDs underneath the screen. Check the LCD connector is connected properly on both the logic board and to the screen PCB. Also check the backlight cable is connected properly (this is one of the two other cables connected to the screen PCB, the other being for the camera). Make sure there is no dust or corrosion around any of these connectors.
Check the logic board for liquid damage if this doesn't work. If you still have the same issue, it will likely be screen related.