The bug
In survival, breaking a shulker box containing item stacks above their stack limit "unfolds" the stacks causing the "unfolded" stacks to be in slots that are above the maximum slot.
Placing the box causes all items in slots > 27 to be deleted.
How to reproduce
Place a shulker box in the ground
Insert a stack of an unstackable item with a count greater than 26 using the /data command (example: a stack of 30 diamond swords)
Break the shulker box in survival, causing the stack to be split past the slot limit
Place the box back, all items that above the slot limit will be destroyed
Linked issues
is duplicated by 7
MC-149868
Broken shulker NTB data in 1.14+
Resolved
MC-162742
Stacks of unstackable items in a Shulker Box overwrite other items when the box is broken and replaced
Resolved
MC-209535
Unstackable items overwriting Shulker box data
Resolved
MC-213254
"Tried to read NBT tag that was too big" kick loop after mining stacked item filled shulker
Resolved
MC-213497
Items stacking in Shulker boxes
Resolved
2 more links
Item stacks above the stack size limit are not supported.