The moss/cobble floor, spawners and chests of dungeons that sand falls into because they spawn on the surface do not generate and appear to be replaced by the sand that fell in. For example create a new large biomes world in 14w06b with the seed "Anubis" and tp to -355,73,1349. There is a dungeon without a spawner or chests (it was a skeleton spawner in 1.7.4). Another example in the same world is at -36,66,161 where some of the moss/cobble floor generated as expected beneath the sandstone where no sand fell in but did not generate below the open area where sand did fall in. Underground dungeons appear to generate correctly - see the spider dungeon at -336,41,817.