• Place a stack of (normally un-stackable) items, such as 27 boats in one slot of a Shulker box
• Fill the rest of the slots with any items
• Break the Shulker
• Place the Shulker
• The stack of boats will have spread out to 1 per slot, and the items in those slots will have disappeared

Here's a video that will better explain the issue: https://youtu.be/BUm92loAnj0