You have just demonstrated "case geometry fault".
In the standard diving position (looking down at about a 45° angle), the center of the diaphragm is about equal with the top edge of the exhaust valve.
Now, the theory part:
When you push a buoyant object below the surface of the water (a rubber ducky), the water pushes back with a force equal to the amount of water displaced with your effort.
In the same fashion, when you look down (or even better, float head down), the air-filled second stage is like a little cup that you've pushed below the surface of the water, where the diaphragm side is about 1 1/4" lower than the top edge of the exhaust valve. If anything were to open the second stage valve, air would trickle out the exhaust.
And in fact, something DID open the exhaust valve: the 1 1/4" of water pressure difference between the part that holds the valve closed (diaphragm/lever/spring apparatus), and the easiest place for air to escape (upper edge of the exhaust valve).
That is why many manufacturers of regulators with the T3's shape prescribe a minimum "cracking effort" (or effort required to crack the valve open), of 1.1". Your reg must have been tuned perfectly!
In most positions you will be fine. In the worst positions you can fix the problem by increasing the cracking effort manually, which is what the adjustment knob does.
It is why my favorite reg is the old Scubapro D400. It has its diaphragm and exhaust valve oriented together, coaxially. In just about any position, your breathing effort can be as low as 0.5"!!
Unfortunately, its exhaust valve is rather small, so my T3 is a better all around reg, especially deep. But I do love my D-series!
No, your reg is fine! As
@buddhasummer pointed out, you can screw in the orifice 1/24th of a turn. Or, just screw in the adjustment knob a touch.
For a discussion that will bore you to tears:
Are Atomics worth the cash?
