I've had an issue wile playing ArmA 3, had this problems on several mods, but mainly on Breaking Point. The problem occours when you swap backpacks and/or equip a west, some/all items in the clothing disappears.
What happens based on my experiance, is that the size of the clothing changes when your total inventory capacity changes. This then leading to items inside the clothing to disappear.
Items can allso disapear from the backpack when equiping a west, this is a more rare occurance compared to the clothing issue. I would thing the two are somewhat related.
You can see this youtube clip to see what happens for yourself.