Glad to hear it may have been an "easy" correction.
You haven't exactly discovered anything "new", though.
I've watched any number of experienced shooters switch to a new platform which has the slide stop lever located differently than in the guns with which they're familiar, and experience early slide stop lever activation.
If the gun has a slide stop lever only on one side, it's often easy to have the shooter switch the gun to the other hand and shoot. If the "problem" is gone, the lever/thumb interface was likely responsible.
This sort of thing is usually more noticeable on the plastic guns which have the levers moved more to the rear of the frame, as well as the Sig P-series pistols. Not always, though.
I remember watching a gentleman shooting a Walther PPK/S one time for a qual course of fire. The mag was dropping out of his gun at least once during every mag load. I watched him and saw something I'd not seen before.
He was a right-handed shooter, using a 2-handed grip with curled his support thumb in an unusual manner. While his support thumb was nowhere near the mag catch button before pulling the trigger, under recoil I saw his curled thumb shift and "precisely" press against the mag catch button ... predictably dropping the slide. Once I pointed out to him what I'd seen, and he changed his grip a bit, the "problem" disappeared. He was a bit testy about it, though, It seems he'd been tolerating the "problem" for a number of years and had simply assumed it was a gun problem, not a shooter problem. The ease of correcting the long standing problem made him feel a bit annoyed with himself.
Then, I had a couple of right-handed shooters experience the slide stop lever not engaging and locking back the slide on empty mags when shooting a P239. I was working with one of them and had him shoot the gun left-handed ... and the "problem" disappeared, only to reappear when he switched back to shooting the gun right-handed. It seems his right thumb was riding the lever and stopping it from rising when the mag follower & spring was trying to lift the lever on an empty mag. Another "easy" correction.
Then, there's been those times when a weak mag spring, damaged follower or a very dirty mag body caused the same thing.
Another thing I've unfortunately seen all too often is how someone manages to reassemble their mags incorrectly after cleaning them. I've seen mag springs reversed or installed upside down (in mags where the springs had a definite orientation and were different at each end), as well as followers installed backwards ... and cause the slide stop lever not to engage properly and lock the slide back when the mag ran empty.
It can be amazing, at times ...
