I realise that we are probably using different regs, but the only time I'v ever seen a reg that wouldn't shut down when the pressure inside the 2nd stage was increased, either by venturi control or shoving a thumb in the orifice, the valve spindle (air barrel) was installed back to front.
I'm not saying this will be your problem, just that supposed expert reg techs often get it amazingly wrong.
I'm not saying this will be your problem, just that supposed expert reg techs often get it amazingly wrong.