I have experienced issues with the “pinky extension” sliding up on the P938 7-round magazines. I first noticed when I was doing magazine swap drills and letting the dropped magazine hit the floor. What was happening is it felt like the “new” magazine was locking into place, but upon racking the slide, it would chamber one round and the magazine would fall out on the floor. This photo shows an obviously exaggerated slippage, so I have trained myself to insert the 7-round magazines with my thumb on the very bottom pushing up against the metal floor of the magazine.
It does not take much, maybe 1/32" to 1/16" sliding up to prevent full engagement with the magazine release.
Edit: This is when I discovered it Sig P938 Mag Release Issues