This is related to MC-148767 in that when you try to get the falling_block to have a Passenger of a falling_block, the second, and later ones, do not become solid blocks.

I thought it was my fault so I was hunting for a mistake when I noticed an odd behavior.  It can be seen in this video:

 https://youtu.be/KpcxiOI0Bjs 

The code used is this:

/summon falling_block ~5 ~1 ~ {BlockState:{Name:command_block},Time:1,TileEntityData:{Command:"/fill ~-2 ~ ~-2 ~2 ~5 ~2 minecraft:cobblestone hollow"},Passengers:[{id:falling_block,BlockState:{Name:command_block},Time:1,TileEntityData:{Command:"/fill ~2 ~-1 ~2 ~2 ~4 ~2 minecraft:dark_oak_wood"}}]}

On that same note, I did notice when comparing 1.12 and 1.13/1.14 that blocks overlap when falling more in the more recent versions and have a suspicion it is because commands resolve quicker.