The bug

I ran through breaking and placing a lectern several hundred times, and Mending, Frost Walker I and II, and Curse books always costed an even number of emeralds. There should not be an odd-even bias, as it doesn't make logical sense and no other enchantments have this.

Steps to reproduce:

1. Spawn villager
2. Place lectern
3. Check enchanted book trade
4. Break lectern
5. Repeat several hundred times

 The cost of a treasure enchantment is never an odd number of emeralds.