Items in the inventory, rifle slots, sheath, holster, and canteen can be easily unintentionally dropped. If said item is in the character's hand(s) and you drag a new item from the environment into your hands, the pistol/knife/canteen etc no longer returns to the designated slot in the inventory and is exchanged for the new item in the environment.

Picked up a lot of items, even while aware of the new issue I unintentionally left mainly my knife behind numerous time.