It's common on semi auto pistols. My Glocks do it if I slam it hard enough. The slide stop takes some of the vibration from the slam and releases.

Does the same happen with an empty magazine? Is it happening on both magazines?

I would say shoot it a bit more. All the springs and magazine catch are new and with time will wear and settle in. 300 rounds should break it in nicely.

Cool that you are shooting! Hopefully carrying too.