Relates to:

• MC-92780

The bug

When an FallingSand entity replaces an existing block with nbt data (tile entity), it does not use the nbt data of the block.

How to reproduce

1. Place a command block and enter something in the command text field
2. Stand on top of the command block an run the following command

   /summon FallingSand ~ ~-1 ~ {Time:0b,Block:"command_block"}
   

3. Look inside the command block
   The command text field is empty

Possible fix

When the game transforms the block into an FallingSand entity, it could copy all specific tags (unspecific tags are id, x, y and z) into the TileEntityData tag of the FallingSand:

/summon FallingSand ~ ~-1 ~ {Time:0b,Block:"command_block",TileEntityData:{Command:"Some commands",LastOutput:"{\"text\":\"No Output here\"}",TrackOutput:0b}}