I can say with confidence that I am not the only person to suffer a sticky valve stem nut. This problem was reported more than once in good old rec.bicycles.tech.

“The deal is that when the tube is pressurized, the valve is forced down hard against the rim, but when it goes flat and the pressure is released, the valve retracts a bit, causing the nut to get tighter.”

—Sheldon Brown, r.b.t 2003