Hello! I'm unsure if this has been addressed yet, but if there is not enough room in your backpack and you attempt to move an item from your inventory in it (with my experience large items like guns or vehicle parts) it will disappear. This has happened to me numerous times. Also, when I die my main weapon is always gone, no matter how soon I get back to my corpse.

This is always been an issue its just how the ARMA inventory system works and will never be fixed in the mod it's simply not possible. Never move an item to a full container.

The corpses not having main weapon is because the main one gets dropped on the ground next to the body. Some players have reported they have been disappearing and the bug has been reported to the dayzmod devs.