Just a thought, the fuel seepage may be the result of the fuel expansion (a boiling effect) from the heat of the engine after the bike is shut off. This also aerates the fuel and allows the float to sink and allow the bowl to become over full.
The fuel in the float bowl will almost boil from a hot engine after it is stopped and no longer running. This is why many people experience gas coming out their over flow tube when they stop riding and park their bike.
A lot of people think this is do to a float level problem, but that's not the case all of the time.
This is why you have a petcock, the petcock should ALWAYS be shut off when the bike is not in use.
Fuel over flow was a BIG problem decades ago on street bikes and that is why the majority of all street models today have vacuum operated petcocks. This shuts the fuel off automatically when the engine is stopped.
Note, street bikes with auto fuel shutoff can still experience fuel seepage on hot day caused fuel expansion in the bowls.
This is my opinion why gas seepage can occur around the mixture screw. Fuel expands do to heat and it is pushed up into the main jet block, allowing the fuel to run back to the mixture screw. The mixture screw does not have a O-ring to seal it to the external part of the carb. It's a wicking effect do to heat and expanding fuel.
Just my opinion :)