The bug

When using enchanted books to enchant items if the books are combined before with another one the cost of applying them to the desired item gets more expensive.

On previous versions, if enchanted books were combined with an item the cost only would be the levels required for enchantments that can be added (lets say 5 levels for a Sharpness V on a diamond sword without any other enchantment) even if they came from mixing books with the same enchantment, now on 1.13 using books that were combined before with other books to get higher level enchantments makes the cost of adding them to items more expensive (Mixing 2 Sharpness III books together, then mix them with a Sharpness IV book and adding the resulting book to a diamond sword without any other enchantments cost 8 levels, on previous versions it would be 5 levels), therefore you get the message "Too expensive!" when trying things like Sharpness V, Sweeping edge III, Looting III, Unbreaking III, Mending diamond sword using books that were combined before to get a higher level, one book per enchantment.

How to reproduce

1. Give yourself an enchanted book with Sharpness II:

   /give @s enchanted_book{StoredEnchantments:[{id:sharpness,lvl:2}]}

2. Use an anvil to combine an unenchanted sword with the enchanted book
   → The operation costs 2 experience levels
3. Give yourself two enchanted books with Sharpness I:

   /give @s enchanted_book{StoredEnchantments:[{id:sharpness,lvl:1}]} 2

4. Use an anvil to combine these two enchanted books into an enchanted book with Sharpness II
5. Use an anvil to combine an unenchanted sword with this enchanted book
   → The operation costs 3 experience levels