I thought initially that it was limited to a 6 lamps rectangle but then I notice that many rows/colums have the problem. I doubt that diode is the culprit or they are multiple diodes failing.
Have you tried the lamp test mode, to see whether you get the same problem ?
It might be the IC that order the lamps that has a problem.
Also you could check that wires are not shorted, by taking the continuity between the lights that are lit. The voltage on the lugs as well.