Follow up on this problem. It was about 50-50, both mags. Kahr sent a new spring. Before installing it, I was working with a private instructor (mainly with my Glock, but finished up with about 25 rounds w/ the P380 practicing pocket draws).
After the first mag (the slide locked back properly) he suggested a different grip for the little gun. Instead of having my right thumb alongside the frame, he moved it to grip over the base of my left thumb. After practicing with it like this on my own, I noticed that there were zero problems with the slide not locking back. Out of curiosity, I ran a mag through with my old (normal) grip style. Slide didn't lock back. Three more mags w/ the new grip, no issues w/ the slide not locking back.
My conclusion - my right thumb was riding the slide release lever, preventing it from fully engaging the slide on the last round. It is a very small gun, and like others have said, that is a little spring isn't the strongest in the world. It doesn't take much to prevent it from engaging the slide. I might end up replacing the spring anyway, but don't want the opposite problem. I'd rather have the slide not locking back after the last round than having it lock back after the 1st! ;D