When in your inventory, but NOT as seen from in your hand or as a dropped item entity, the shulker box item uses the top texture on all sides. (Compare to the block when placed in the world.)

Steps to reproduce:

1. Get a shulker box item.
2. Look at the item in your inventory (not as it appears in your hand or as an item dropped on the ground).
3. Notice that it uses the same texture on all sides, unlike how it appears in your hand, as an item entity, or as an actual block.