Reproduction Procedure：

• Enter the game.
• Switch to creative mode.
• Place a shulker box and fill it with items.
• Hold down ctrl and press the middle mouse button.
• Hold down ctrl and press the middle mouse button to copy a labeled Shulker Box.
• Throw them out one by one.
• Pick up all the thrown Shulker Boxes.

At this point, you will notice that some Shulker Boxes have disappeared from your inventory.

I speculate that it's because the location shown in the attachment is shallow copy.