FallingSand ignores rotation tag

Please provide the exact command that you've used.

/summon FallingSand ~ ~ ~ {Rotation:[45f,0f]}

Blocks cannot be rotated. The rotation tag is for mobs, projectiles, vehicles etc.

FallingSand isn't a block. It's an entity. The reason they don't rotate is because rotating the block model on the entity wasn't implemented. This isn't technically a "bug", as Mojang never intended to allow FallingSand entities to rotate. While it would be nice to see it in game, it's more of a "requested feature".