The bug
Breaking a shulker box with a loot table causes it to generate the loot despite it not being opened.
How to reproduce
Use the following command to place a shulker box with a loot table
/setblock ~ ~ ~ white_shulker_box{LootTable:"chests/simple_dungeon"}
Break the shulker box
Pick up the shulker box item and hover over it
Confirmed for 1.13.1-pre1