When the "Villager Trading Rebalancing" toggle is on, librarian villagers will never sell max level common enchanted books (except for one level only enchantments). In java common enchanted books can be max level enchanted so i believe this is unintentional.

This is easier to test on savanna villagers since you can only obtain curse of binding (1 level) and knockback (2 levels). You will see that you can only get curse of binding and knockback I.