Steps to reproduce:
-Put an item into an urn
-Close the urn, saving the nbt
-Open the urn from your hotbar
-Remove the item from the urn
-Press the appropriate number while hovering an empty slot to move the urn there
-Close the urn, saving the nbt to nothing (or a random item, which is worse)
Results:
You now have the item in your inventory and inside the urn. You can also get an item that won't stack properly.