I know what youre both talking about, and youre talking about different things. I have the xdm 3.8, my compact mag came with that plastic bit which i believe is meant for dry firing without holding down the slide catch. Now the issue with the slide lock disengaging when you put in a new mag is due to the catch not being flush with either the slide or frame. Because of this i think it just barely engages the slide keeping it back, so when its slightly knocked ie when you put in a new mag it moves out of place allowing the slide to move back into battery. Check if yours has any horizontal movem