One other way to approach it that i just thought of, is that if you only want items to spawn above your ground tile, then while spawning your items, use a "pick by evaluate" to only pick tiles with the bitwise value that equals your standard "ground" tile, then subtract the tile height (eg: 16). Thus they will only appear above the ground, and centered in the correct location.
pick all tiles = tile.bitwise = "ground"
randomly pick how often a item should be created eg: 10% of the time
for each tile -> spawn item at tile.x, tile.y-tile.height