When exiting your inventory, (E or Esc.) the item you were hold drops out. I don't know if this is intentional, but when you have an item in a GUI interface (Crafting Table, Furnace, etc.) the items will go back into your inventory without going onto the floor. However, when you click on an item and leave it drops. I don't remember this happening in 1.13.
Only tested in Survival Singleplayer.
Related issues
Comments

Duplicate of MC-96782
This isn't where to do so. Please delete your comment from my bug report please.