Upon opening a chest found within a library in a stronghold i saw a enchanted book with the enchantment loyalty 3 which cannot be gained in survival
Along with this in a survival world where i was afk fishing i got books of the following enchantments
unbreaking 2 and unbreaking 2 on the same book
and protection 3 and protection 3 on the same book
I've tested the stronghold loot chests 10 times and only twice it occured
and afked fish for approximately 2hrs and 3 books had enchantments which aren't possible in survival
The easiest way to reproduce it is with fishing.However it may take anytime between 10minutes to 12hours
Cannot reproduce.