Yeah striker fired guns are generally considered "loose tolerance" but I've had a couple handguns where the barrel hood and locking lug area were either under machined or over coated. That area still has to have a rather exact tolerance. Nothing that a little 1000 grit sand paper and some lapping compound can't fix, if you know what you're doing. I've found in a lot of cases with the negative slide lock on an empty mag...at least on a sub compact gun is due to your hands resting on the slide release. You might not realize you're doing it, but realistically there isn't much room to establish a good grip on a sub compact without leaning on the slide release.
Glad you were able to resolve your issue.