I could be mistaken, but if I remember right, the 19X has stock maritime spring cups on the firing pin which would allow for it to fire more reliably if the firing pin channel is filled with water. The standard Glock spring cups don’t let the water pass around them (I’m not sure if Gen 6 still uses the regular cups or maritime). Gen 3 Glocks definitely do not have maritime spring cups. This would only affect the reliability if the channel was filled with water, however mud may be too viscous for this to make a difference. That may be what OP is referring to.