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".