The bug

When a FallingSand entity breaks, for example because it landed on a torch, it transforms into an item. However this item only uses the id and the data value of the FallingSand but not the data of the TileEntityData tag.

How to reproduce

1. Use the following command

   /summon FallingSand ~ ~1 ~ {Block:"command_block",Time:1b,TileEntityData:{Command:"Test"}}
   

   The command block will have the word "Test" as command

2. Use the following command

   /summon FallingSand ~ ~1 ~ {Time:1b,Block:"torch",Passengers:[{id:"FallingSand",Block:"command_block",Time:1b,TileEntityData:{Command:"Test"}}]}
   

   The dropped item shows that it does not store NBT data and when placing it, you will see the command text field is empty

Possible fix

When the item gets created, the game could transfer the data from the TileEntityData tag to the BlockEntityTag in the tag tag of the item:

/summon Item ~ ~ ~ {Item:{id:"command_block",Count:1b,Damage:0s,tag:{BlockEntityTag:{Command:"Test"}}}}