There was a small defect in a few of the intial production runs that allowed it to fire when dropped, but it was not an easy condition to achieve. SIG changed the design anyway despite the crazy amount of factors that had to come together to create the event, and all subsequently manufactured 320s are safe as can be (including every single M17/18)
if you own a very early one check the recall notices, if it's not a very early one nothing to worry about.