Villagers, exchanging 2 items for one, have their second required item in trade deleted after reloading the world.

Villagers, who exchanges two items for one (for example, librarian who gives you enchanted book for normal book + emeralds), wants only the first item, when you reload the world/load that villager from the previous version, making it possible to enchant items or transfer gravel into flint without paying. They will also generate that trade again, in unbroken state, that will broke as the original one upon reloading.