The librarian in my village has a book of mending as its first offer, which I traded six times. When I came back later to by more, I couldn't select the offer with mending. Everytime I tried to select the first offer, the second one (bookshelves) was being selected. As can be seen on the first screenshot, the librarian has an enchanted book in his hand when I select the desired trade. But in the trading GUI he instead offers bookshelves. When I complete a trade with the first offer, I get bookshelves and pay their price, instead of the price for the enchanted book. Furthermore the screenshots show, that the following offers (three and four) work as intended. Only the first offer is unselectable.
Thank you for your time.
(english is not my first language, so if you can find any faults, you can keep them 🙂 )
Emeralds + Book = Enchanted Book