The bug

When using /setblock or /fill to replace a block entity, this keeps the old NBT (if no NBT is specified), but clears the inventories, even if the command fails.

Additionally, the command will fail if the specified block state matches the existing one, even if there is new NBT to apply. The result is only inventories clearing.

How to reproduce

• Example 1:

  /setblock ~1 ~ ~ dropper[facing=up]{CustomName:"\"first\"",Items:[{id:stone,Count:1b,Slot:0b}]}
  

  This places a dropper as expected. Now run the following command:

  /setblock ~1 ~ ~ dropper[facing=up]{CustomName:"\"updated\"",Items:[{id:stone,Count:1b,Slot:0b}]}
  

  Expected behavior: command succeeds (because NBT differs), dropper updates name.
  Actual behavior: command fails (because the blockstates are the same), dropper does not update name, inventory clear.
  Note: Leaving off the NBT in the last command has the same effect.

• Example 2:

  /setblock ~1 ~ ~ command_block[facing=up]{Command:"first"}
  

  This places a command block as expected. Now run the following command:

  /setblock ~1 ~ ~ command_block[facing=down]
  

  Expected behavior: command block is empty
  Actual behavior: command block contains the command "first".
  Note: including NBT in the last command successfully places a new block as expected.