How to reproduce:

1. Trade melons (or anything else for that matter) with a villager.
2. Sell him out of the melon while still holding melon.
3. Close the villager GUI.
4. even though you just sold him out, he will keep holding the emerald (which you get by giving him the melon).
5. hold something else (no melon).
6. His hand-item (emerald) will disappear
7. swap back to melon
8. he wont hold the emerald anymore