Not owned 8fields kit, but I've never had a mag, pistol or rifle, fall out of those kind of pouches, so it's probably safe to assume you never had it adjusted tight enough, not a criticism just an observation.
As a young squaddie literally 4 decades ago, one of the first things we were taught was once your kit was loaded up, have a good jump around in front of your mates, looking for anything that might make a noise & give you away, & that everything is secure.
Good rule of thumb before your next game.