Villagers won't stop giving me bad trades after I cured one

Ever since I cured a librarian the whole village is giving me bad trades!

I cured a Librarian to get better trades from the villager. I had a zombie turn it into a zombie, then I cured it. When it was cured and I opened the GUI it increased the trade of paper for emeralds from 24 paper to 31 paper. (I did not punch the villager as it was a zombie nor did I ever punch any other villager in the village.) After the day had past the trades went up even more, it eventually took 64 paper for a single emerald. While this was going on all the other villagers started to give me bad trades as well (I think that was the gossip feature). I am just confused as I have done nothing to the villagers to make them increase their trades, punched them, took away their stations, ect. I have been trading with the villagers for days and they are all master now yet the trades are still the highest they can be. I think this is a bug but if there is something I'm doing wrong, what is it?