Summoning an entity with a falling_block as a passenger causes the falling_block to initially spawn at the entity's feet and then shoot upwards.

When summoning an entity with a falling_block as a passenger, the falling_block initially spawns at the entity's feet, then shoots upwards to where it should be. Example command:

/summon minecraft:armor_stand ~ ~ ~3 {NoGravity:1,Passengers:[{id:"minecraft:falling_block",Time:1}]}

If the entity(armor stand in this case) is spawned on the ground, the falling_block instantly becomes a block because of its initial starting position at the entity's feet.

The attached .gif shows frame-by-frame exactly what happens.