Whenever a /summon command is used to turn a shulker box into a falling block, the game not only spawns the falling block of an empty shulker box, but it also spawns a shulker box item with the old contents inside. That means a new shulker box is created in the process.

How to reproduce:

1. Place down a white shulker box, and put items inside.
2. Run the following command while standing on top of it:

   /summon falling_block ~ ~-1 ~ {Block:minecraft:white_shulker_box}

3. A shulker box item with the items inside should have dropped, and the falling block should have landed and placed an empty box.

I'm not sure if this is the intended behavior, but it definitely does not seem right.

What might be the right way to do it is to put the data of the shulker box into the falling block's TileEntityData tag directly and drop no items. Another way might be to simply drop the contents of the box as items, without the box, and let an empty box fall.