This used to be a glitch in 1.14 with all entities, however I saw a bug report for that and it appears to be fixed. This glitch still applies to falling block's meaning that when a falling block riding another falling block lands, the block on top will be destroyed. This issue makes it impossible (as far as I have understood) to make One command creations.

The issue can be replicated by the following command:

/execute align xyz run summon minecraft:falling_block ~.5 ~ ~.5 {BlockState:

,Time:1,Passengers:[{id:falling_block,BlockState:

,Time:1}]}

The top command block will just pop off, howver in any version of 1.13, this glitch would not occur.