Adjusting the tension of the field strip screw can change the angle of how the valve sits with respect to the body. This can cause the bolt to be pushed to one side of the powertube more than the other. Pushing the bolt back a bit by applying tension on the trigger probably realigns it a bit stopping the leak. Check that the pim (threaded protrusion that the grip frame screws into) on the bottom of the body is not bottoming out in your rail.
The bumper and bolt spring won't cause the bolt to sit too far forward. The combination of the sear position and valve position determine where the bolt rests. The spacing of the rail (sear pin to field strip screw) determine the exact distance.
Another thing to try for the purpose of confirming the cause and fix, would be to turn out your powertube tip about 1/2 turn. This will allow the carrier to sit farther forward. Install the valve and air up the gun. Check to see if it still leaks while at rest. You can cycle it like this just for checking operation, but don't use it like this because the powertube tip will come loose.
Except for the Automag in front, its usually the man behind the equipment that counts.