Here's a puzzler. On my American Standard Strat the G string is not picked up on the middle pickup but is picked up on the bridge and neck pickups. When I play the G string on the middle pickup it's just like playing it unamplified. Try to figure that one out!
You've presented us with a puzzler. It's only a guess on my part, but it sounds to me like the magnetic pole on that string has lost it's charge. You could test this by touching a common nail (iron) to that pole to see if it's attracted. If that proves to be the case, you may be able to remove the pole piece and recharge it magnetically by stroking it with another strong magnet. If you're not very skilled this way, then I suggest you taking it in for repair by a qualified technician. I wish you luck.