Top half of screen dimmer

I got a used LG tv (55SM8600PLA) from a friend and when I turned it on I noticed that the top half of the screen is dimmer than the bottom.
He said that he didn't notice that when he still used it a few days before.
so either the issue happened during transport in my car or he just didn't see it.

To fix it I already tried replacing the led stripes (the tv is edgelit from the bottom) but that still produces the same issue.

I found the service manual, and the few voltages that were listed there
130V for the backlight which seems fine
mainboard voltage is only 13.0V when on, instead of the listed 13.2V, not sure if that is within the margin or not
one other voltage that was listed is one that goes to the panes, which is 1.9V instead of the 1.8V listed, also not sure if that is within margin?

Anybody knows what could be the issue here?