Sometimes (in desert caves for example) the sand generates with no blocks under it.

When sand doesn't have any blocks under itself, it is supposed to fall, until there will be a block that stops the sand from falling.

But when the sand is generated as a ceilling of a cave, it stays in the air, but it is supposed to have block under it, or it should fall

You can see it by finding the desert biome, and a cave. The non-falling sand does not generate everytime though.