What I expected to happen was...:

In both situation, sand will return to a solid block

What actually happened was...:
only the falling block with a non-air under it will turn into solid block

Steps to Reproduce:

summon/place a falling block with something like 3/4 in air and 1/4 above a solid block (with a command block it's :   )

summon minecraft:falling_block ~ ~3 ~0.75 {Time:1}

and if next to the solid block there is :

  -block (torch, full block...) : sand turn it into solid block 

  -air : sand stay as a falling block and it drop 

 (note : I used the same setup as MC-165728 but it's only because I had found with this bugreport)