Any idea why some of my leds are dimly
lighting up when it's supposed to be off,  because i'm getting a tiny
bit of leakage current from other positions in the matrix?

Suggest you give a bit more info such as which machine and which lights i.e playfield or backbox.

My two cents

Ok as you can see in pic 2x 4x & 6x are dimly lit, there are a few more...

I am no expert but it could mean you have a ground somewhere.  In other words you seem to be right about a leakage....  I would start at the source of the light that is supposed to be off and work my way up to see what could cause it, my bet it that something is touching something that is not supposed to be touched by that something ( should I add something in there?)

At least it is on small power, I was involved on a ground with 440 volts.....took a while to track down!!!