I realise that you think it isn't causing it
I'm not "thinking" GSX is not causing it. It's NOT causing it.
(I don't see why either), but I'm 90% sure it is causing it.
That you don't see it or how much you are sure of it, doesn't change the fact that GSX is not causing it and it CAN'T cause it.
I have confirmed that I do have all the power sources on (APU, EXT PWR).
You must have overlooked something. Fact is, the instruments are not getting power, and there's no way GSX could affect this.
It only happens when I use GSX.
No, it's not. You probably haven't noticed before, because you never turned off engines when parked before, GSX is just asking you to do it, but the rest of the airplane operates EXACTLY as before. I'll repeat it again, this time for the last time, GSX DOES'T CHANGE ANY AIRPLANE VARIABLES, IT ONLY READS FROM THEM. This means IT CAN'T cause this, it's IMPOSSIBLE.
I've tried it about 10 times.
Since nobody else reported it, it's fairly obvious you are doing some kind of mistake, 10 times.
However it did work OK for about 4 flights, then stopped working.
Even more proof (not that any was needed) that it wasn't GSX, since GSX hasn't been updated for a while.