Well, the only way I can see the gas is even getting to that hole is around either the piston or piston stem o-rings, so it has to be related to one of them.
Only things I can suggest are:
rebuilding the reg yet again, checking nothing is preventing the seal on the piston stem oring in particular. put a little grease on these orings too.
When it's assembled, screw the adjuster all the way in real tight, that should help the piston stem seal before you put gas through it
add gas as quick as possible to try and seat the oring and also once the gas is oon full, screw the adjuster out and back in a couple of times quite fast to get the seal.
Or
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