Trading becoming expensive without reason

Villagers are demanding more emeralds and items for trading without any reason and it just keeps on going. I cured them from zombie villagers and their trades were super cheap and now it seems like trades are returning slowly to original price. Not sure if its bug or a feature, there's no information on wiki page. Not sure if lack of beds or sunlight has something to do on this situtation. I cured this farmer in the picture below and he/she gave me discound but now the discound is gone. I haven't hit any villagers, iron golems or pandas