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:
- Get a shulker box item.
- Look at the item in your inventory (not as it appears in your hand or as an item dropped on the ground).
- 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.