On most modern auto pistols, the back side of the slide lock has a tab (lug) that projects in to the cutout area in the mag body. Once the last round is fed from the mag, the follower will engage the slide lock’s tab and put upward spring pressure on it.
Just about all modern auto pistols work this way to lock the slide to the rear once the last round is fired.
On most handguns, including the Springfield XD and XDM, the slide lock protrudes in to the mag far enough so there is no chance of the mag follower failing to engage it once empty.
With the XDS, the triangular protrusion barely catches the edge of the metal plate that it is supposed to catch on the mag follower. A little but of sideways pressure or a slightly off-center and push from the mag spring, and fail…
When the mag is dropped, the slide closes on an empty chamber.
I’ll be calling Springfield on Monday. I have five mags and this is repeatable on all of them. Two x 5-round mags came with the gun and I bought three more 7-round extended mags.
Back in the 90’s I had a problem with a Springfield 1911. I sent it in and received back a masterpiece of reliability and accuracy. This is what made me a long time customer of Springfield Armory. If they stand behind the XDS and fix it the way that they stood behind and fixed their 1911 nearly 20 years ago, I’ll be one very happy camper.