Cured villagers appear to reset the disounted trades. Basically when you complete "Hero of the Village" the game defaults the villagers back to normal and discounts them.

I have cured which normally would trade me 1 emerald and a book for a enchanted book.

Im not quite sure what caused my villagers to go back to normal but it might be because Hero of the Village messes it up and puts the villager trades back to normal and discounts them that way. Because cured villagers cant really have a better discount because they are already discounted.