/execute at @e[nbt={shake:1b}] align xyz run summon falling_block ~ ~-1 ~ {Block:"minecraft:sandstone",Motion:[0.0,1.5,0.0]} throws sandstone block under the arrow, but when block lands, it slides to ~-0.5 by x and z.
[media]
and falling block just stays there not droping or transforming into normal
/execute alignapparently floors the x, y and z coordinate which means the position is the -x, -y, -z corner of the block position. You would have to offset the x and z coordinate by 0.5 to summon the sand centered.