Here is a 'maybe" place to look.
As the last round clears the follower,the follower raises and engages the slide stop.That ordinarily is how the slide stays open on the last round.
Could be maybe something like a weak plunger spring or something about your tolerances is letting the slide stop find something to drag on,like your disassy notch.
On Colt single stack mags,there is a little dimple raised on the follower to help hold the last round from scooting forward during recoil.Do your followers have that dimple?
You can't go wrong replacing your mag springs with Wolfe +10 % springs.
While you are at it,recoil spring could play a part if it is weak.How?
If the slide is smacking the frame with sufficient violence,it will help knock the handgun back out from under the last round.