Currently when placing backpacks on the ground by default you can not group them together as a pile. If you do that they will appear all spread out over the location.

You can also not place backpacks in houses (even with elevation). The backpacks usually end up outside of the house or under the floor somewhere. The backpacks also conflicts with other objects on the map. If you place it near an other object the backpack will spawn a couple of meters away from the object.

No doubt this is due to the fact that the backpack has physics now. I really would like to see a checkbox in the object where physics can simply be disabled.