Your floating problem isn't caused by the objects or the way people are placing their spawns. The problem comes from line 85 of FSS_fnc_spawn_loot_inBuilding here:
if (_lootPosition select 2 < 0.5) then {_lootPosition set [2, 0.5];};
Essentially if your building has a surface low to the ground where you're trying to spawn loot at, then its raising its position above the surface.