Suzuki 2 Strokes

Hello, today I did a pressure test on my freshly rebuilt GT750 engine. Crank rebuilt with all new seals, new cylinder base and head gasket, crankcases sealed with grey THREEBOND.
I sealed the exhausts and put a plug into carb manifolds with attached pressure gauge and pump. Target is to hold 5 psi for 5 minutes.

The left and the right cylinder worked fine, no leaks. The middle cylinder though drops more than 1 psi per minute. Spraying soap water did not reveal the source of the leak, and I cannot hear it.
Do you have any idea what the likely cause could be?

It could leak into the gearbox at the primary drive between middle and right cylinder, but how could I confirm that? Do you think it would suck gearbox oil then, and could it eventually also draw air from the gearbox?
Or could the head gasket leak from cylinderhead into water jacket? Is this why SUZUKI recommends to add some bar’s leak into the coolant?

And oh, I replaced the SRIS valves with solid studs. They dont leak either.

I rebuilt quite a few of other 2T engines and I was always able to easily identify and fix any leaks. Here I dont know what to do. Leave it as is?

1 psi per minute is fairly slow so I would just assemble it and run it and then take off the head and look at the piston tops to see if their proportions of blackened top to clean top is about the same on all 3 pistons.