Put the summary of the bug you're having here

When trading for enchanted books with a villager and shift clicked the books in to your inventory you will also buy cheaper books for example if you buy a mending book for 36 emeralds but you put a stack in and the villager has a cheaper book trade knockback 1 for 10 emeralds than you will buy 1 mending book and 2 knock back 1 books

What I expected to happen was...:
Describe what you thought should happen here
I was expecting 1 mending book to go in my inventory
What actually happened was...:
Describe what happened here
1 mending book and two knockback 1 books went in my inventory
Steps to Reproduce:
1. Get a villager with an expensive book trade and a cheap one
2. Buy the expensive trade but put a stack of emeralds and enough books in
3. Shift click the enchanted books out of the villager GUI and into your inventory