Is he right-handed?
Is it only happening when he's shooting 2-handed, not 1-handed?
If so, does he favor a 2-handed grip where he aligns his thumbs along the slide (high/neutral grip)? The support thumb can shift so it's pressing downward against the slide stop lever under recoil
. This is almost always not felt or noticed by the shooter.
If he's right-handed, does it happen when he shoots the gun left-handed? If not ... that's a clue.
As a LE firearms instructor, I've watched at least my fair share of right-handed shooters exhibit grip techniques that often induced failures-to-lock-back on empty magazines, or early slide lock with ammo remaining in the mag (even using guns with forward-positioned slide stop levers), simply because their grip techniques put their thumbs in the wrong place, at the wrong time, during recoil/cycling.
The shooter influence is often a good place to start looking, since more than 95% of "pistol problems" can turn out to be "shooter problems".