When trading with villagers in survival mode, sometimes the item in your hand disappears. I have lost two Lure III, Luck of the Sea III rods, a Looting III, Fire Aspect I, Unbreaking III diamond sword, and a Fire Aspect II, Knockback II diamond sword. I would do more "testing" to see how to recreate the issue, but I'm getting sick of loosing all my best equipment. I think the issue is when you overload the item box to trade for emeralds multiple times and then take back the remaining items.