To be sure you're not the only one, same thing happened to me. When I pick up or equip the Mk 48 Mod 0, it will drop my backpack. And when I want to pick up a backpack again, it will drop my Mk 48 Mod 0. In both cases it's completely gone, so you cannot pick it up again from the ground. Tested it with several weapons, but only the Mod 0 has this problem. Strange thing is, that it worked a few weeks ago.