A small amount of stamina seems to get used erroneously:
Tested for rifles, handguns and launchers. Strangely, the weight of the weapon appears to determine how much of a stamina hit is taken.
This doesn't necessarily have a huge impact on gameplay; however, it is rather annoying and jarring to see the stamina bar flash obnoxiously whenever a magazine is emptied whilst standing still.
Arma 3 Dev 1.53.132967
Actually, you stamina doesn't decrease. When you deplete ammo (or drop something) your load lightens, so the maximum stamina increases and you start regenerating to get it to max.
But you're still right, it is a bug.
Oh, that makes lots of sense! Definitely left me very confused at first, though. I wonder, should you not instantly gain that difference, rather than having it regenerate to the new maximum?
If stamina, in this context, can be thought of as a representation of your total potential max-exertion over time - inversely proportional to the total weight you're carrying - then an instant reduction in weight should result in an instant increase in your potential.
I might edit this ticket when I get a chance to better reflect what's actually happening.