When having a Locked Shulker Box and breaking it the Lock Tag will be removed from the item.

How to Reproduce:

Step 1: give yourself a Shulker Box with the Lock Tag; /give @s shulker_box{BlockEntityTag:{Lock:"Key"}}

Step 2: give yourself a Key to open the Shulker Box; /give @s tripwire_hook{display:{Name:'"Key"'}}

Step 3: Place down the Shulker Box

Step 4: Hold the Tripwire Hook renamed Key and open the Shulker Box (Trying to open the Shulker Box without the Key will not let you)

Step 5: Break the Shulker Box and place it down

Step 6: open the Shulker Box without the Key then close the GUI and open it with the Key as you can see the broken Shulker Box can be opened without a Key