MC-211283 [https://bugs.mojang.com/browse/MC-211283]

Empty shulker boxes that have been placed and then broken retain unneeded NBT tag that stops them from stacking with unplaced shulker boxes.

To fix this, simply remove this NBT tag. The tag is placed on a shulker box that contains items, but when emptied it shows an empty tag instead of deleting the tag entirely. Alternatively, give crafted shulker boxes this empty 'items' tag.

Call this "ShulkerExtraNBTFix"

I would propose that whatever fix is added also allow pick-block on empty shulkers which worked in versions before the nbt tag thing was added.