To reproduce:

1. Be in survival
2. Craft 2 shulker boxes
3. Place both
4. Mine one
5. Switch to a non-shulker box hotbar slot
6. Pick block on the placed shulker box
7. Observe that your selected hotbar slot did not change to the shulker box in your inventory

This is likely because placed and mined shulker boxes have some nbt data with them, especially if there's items inside (though this bug also applies to empty ones).

I expected pick block to switch my selected hotbar slot to any of the shulker boxes of the same color in my inventory, even regardless of wether the contents match.