Sorry, came from a pub, not wise to do any re-testing now, but from what I recall, after reported was also seeing it inconsistent. The most often happened to Sam with a bat (2 move AP, 9 Max & per Turn AP). Think for a moment crossed a mind if it triggers more often for higher attack AP costs, but that's prob. misleading.
Edit: Oh!... Maybe add Movement AP to derived stats section? (must admit that last unfilled space has been driving me a bit nuts, a tiny bit, not much of course)