So, I have a villager with two enchanted book trade. First trade being mending which was originally 36 emeralds. The second enchantment unlocked was piercing. The mending eventually dropped to 24 emeralds and the piercing to 1 emerald. My issue is any time I try to purchase the 24 emerald mending it gives me piercing and then charges me for the piercing price. I have the 24 emeralds in the slot. I shift click the mending trade option. Also when I curse over the book in the output slot it says mending.