Weird Science

Ok, so I've found the problem with the lightboard after extensive search - When using two different power voltages (sources?) it doesn't work.

Why?
I honestly don't know.

I've double checked the hardware and rewritten the software, nothing seems to make a difference. I've done no change in the circuit, but simply removing the +12V and attaching a cable from the +5V input to the +12V makes it work perfectly. The both grounds are tied together as well and has always been. So the circuit itself should be alright, I guess?

My best guess now is that somehow power leaks into the "on-switch" of the 12V MOSFET since it's not connected to the same power chain as the rest normally, and therefor possibly allows the current to flow differently there. Putting diodes could possibly remedy it. But there's really no room on the board for extra parts, meaning some ugly Frankensteining of parts on top of parts etc.

I'll keep looking. Nobody wants weak 5V flashers... :C

Edit - 

Ok, I think I'm on to something here. And if so, it's a hardware error by design (by me). 
I'm connecting the gate to ground to keep it from floating, but that is how it's done for N-Channel MOSFET's, not P-Channel as those are active when grounded/low. So that'll have to change and will probably help a little with the ever so slight ghosting issue. 

Secondly; 
I thought the VGS value of a MOSFET was some kind of fixed requirement for turning it on. Well, it is - but it also depends on the load. So in this case when using logic level voltages (i.e up to 5V) the gate can be properly held shut by the shift register as the voltage is less or exactly the same as the gate voltage. In comes 12V and I'm suddenly short 7V to properly close the gate using my 5V, and the circuit is held open. Imagine closing a door standing in a river compared to closing a door in a shower, the pressure of the river is just to big to properly close the door and water will leak through etc. 

So I need to first fix the resistors from pull-down to pull-up, and then I guess add a N-Channel MOSFET on top of the P-Channel to act as a valve for opening/closing the P-Channel and thus the heavier load. I'm so glad I've moved the board to an easier to reach location!

1) Colors are off and the light numbers doesn't correspond with the correct channels yet, but you can clearly see there's no wrongful "double" triggering here.