I added most of the new items from the newly released Marksmen DLC to my storeconfig.sqf. It would seem that once any of these new items (guns, optics, suppressors etc)is placed into a crate (vehicle ammo crate, mission crate, empty ammo box etc). They cannot be removed.

I have placed non marksmen dlc items into the same boxes and I am able to remove those items with no problem.

I have also tested both new and old items out with vehicles but there does no seem to be any problem with that.