When you create a villager from a spawn egg and you go to make a quick purchase by holding down the shift key, it will purchase as many enchanted books that the intended book will allow and what ever left over emeralds you have will be spent on less expensive enchanted books.

For example say an Enchanted book cost 35 emeralds from a villager, but the villager also has another enchanted book for the cost of 10 emeralds. With a stack of 64 emeralds, you will purchase the 1 enchanted book that cost 35 emeralds, and the remaining 29 emeralds will then be also spent on two enchanted book for 10 emeralds leaving you with 9 emeralds and your inventory will have 3 books, the one that cost 35, and 2 that cost you 10.  This only happens when you create the villager from a spawn egg. I have only tested this in 1.16.5, but it is doing this in single player and on the realm.

The result of this bug is causing unwanted purchases of unnecessary items.