0.C-g28db51a - 0.C-19534-gec2b8d9 (tiles)
It seems like the windows delete items. When window break no broken glass or curtain if you kill zombie on them the bodies disappear and if you drop something on them the item is gone.
It isn’t just window it is rough terrain that usually pushes things off of them so window, fences, young trees, If something ends up on these tile such as you killed something on it, broke a window, or tossed something on it, instead of pushing the item off it disappears.
Similar happened with me, I could not drop my RM11B scout rifle to the empty floor (or any other tile) with the Advanced Inventory Menu because the “destination area is full”. If i dropped it with the normal inventory menu then it was gone.