Villagers sometimes loose their workstations after trading with them, and then upon logging out and back into a server the trades reset. For example, I had a Villager who traded Thorns III for 16 emeralds and a book, when the bug was happening I could not trade with the villager, and when I logged out and back into the server the trade was aqua affinity for 7 emeralds and a book. Bug happens at random as far as I can see, But is more consistent when villagers are close together with multiple of the same work station, in villager trading halls.