What I expected to happen was...:
After trading, I thought that the stacked lava buckets would be able to be placed down, one by one.

What actually happened was...:
When right clicking the stacked lava items, the bucket changed to only one item. I lost all my other buckets of lava.

Steps to Reproduce:
1. Get a villager leveled up until they let you trade lava for something.
2. Go to trade with them. Select the option to trade lava for something. (The buckets will stack)
3. You don't need to trade, but take out the buckets. They should still be stacked.

4. Put them in your inventory.

5. Go to right click it on the floor and then you will notice all the lava buckets turned into just one, losing the rest.