After unlocking a spellbook trade for a librarian, as well as after first viewing the initial spellbook trade provided by a librarian, after I leave the game and open it at another time all spellbook trades are set to be for the protection 1 enchantment. So if I have a bunch of librarians with 10 different spells available, if I leave the game and come back I will have 10 different offers of protection 1 instead.