I went to the stack in the Large Cabin House. (the second room with the wooden house, stack, bed and closet)
(https://dayz.gamepedia.com/Large_Cabin_House)

Made a fireplace in the stack and put in some sticks in and ignited it.
I had a second backpack in my hands with stuff inside (shovel, some sticks, waterbottle, burlap, saw)

I stand closer to the stack/chimney to warm me up, charakter was freezing.
When i put out more sticks from the inventory to keep the fire burning (drag and dropped from the inventory to the left corner in the "Vicinity" field.)
First the sticks would just hang in the air (well thats not the problem, just looks funny, flying sticks =D )

But when i accidently dragged my backpack to vicinity it disappeared.
I searched inside the house and outside the house (also corners) but coudlnt find it =(

maybe it bugged/dropped down the floor/under the house or on the roof / or in the chimney

copy from reproduce:
stay very close to the chimney and drop a backpack (it was a green Marlin Dryback), via Tab into the Vicinity (i had in the Vicinity field the Fireplace with 5 sticks, 2x 5 sticks, and 3 more sticks (13 in total) in the vicinty.
i have an mountain backpack (full with survival and work utilities, nails hammer, etc...)