If a FallingSand entity is summoned on a empty space/a coordinate that doesn't have that block already, it will not render.

Command used: summon FallingSand ~ ~4 ~-2

y ~4 does not have a glass block, so no falling block is shown, however a block appears when it has touched the ground.

If I put the y coordinate to ~5 It will take the glass block above and make that fall. That is shown.