After Curing a Librarian Villager, It reset their new trade but discount still there.

When I cure the zombie librarian villager which I want to get discount of Protection VI, so I waited until the villager cure. After the Librarian cured, then suddenly villager being reset his job and become librarian again and it reset the trade which mean I lost my Protection VI offer but the discount is still there. It happened like several times when I'm trying to do it. bug happened in my private server.