it's not a problem. just seems to be a quirk of the double-stack mag. when it's fully loaded, the rounds line up in such a way that they're all forced against each other except for one round. this round doesn't have the other rounds touching it with any pressure, so it wiggles back and forth. on my pt140, it seems to be round 8 or 9.
it might actually be round 10, as the follower is forced against round 9 and isn't putting pressure on round 10. round 8 is pressing against round 9, so it doesn't press against round 10. if you take one round out, the geometry changes and all rounds are touching each other.