I have two PA800s and they both have a very long funky yellow streak on the inside of the screen. At first, my older board (non warranty) developed this, so I took my NEW board out and after 3 months, it did the very same thing
Zuki ... he said LED not LCD. There's a huge difference.
Your problem is something silly like you have the contrast too high or the temperature in the room has changed. All LCD displays regardless of make or model will have a higher contrast when they are cold, and a lower contrast when they are warmed up from use or hot from the room they are used in.
It's totally normal. Hell they all even look different from what angle you look at them.
Your other issues are probably smoothing silly also. That's why KORG Forums exists. If you have a problem, share it with others and it gets sorted. KORG even respond to anything that requires their attention.
Regards
James