Before you go and do a refit to the slide stop, check your pistol for an empty chamber, insert an empty magazine into the well, then rack the slide hard. Did the slide stop seat fully into it's notch in the slide or did it just seat part way? If just part way, maybe that's why bumping the magazine dislodges the stop allowing the slide to trip. Could also be caused by a tight fit between the slide stop and the frame restricting full movement of the slide stop which likely will disappear after you break in the pistol. While you're at it, compare the magazine follower height with all of your magazines. Do any of the followers appear a bit lower than the others? Could you be limp wristing the last round? Please do not take this as criticism of your pistol or shooting technique. My intent is just to try and help you locate the source of your problem. By letting us know how this turns out, someone will come up with a fix.