Villager discounts change sporadically

I know this was an issue in previous versions, and it was claimed to be fixed. After trading with a villager for a while, they are supposed to give discounts. And sometimes they do, but occasionally I'll leave the game and when I rejoin, the discount will be gone. For example, a master level farmer will give an emerald for 3 melons normally, maybe they'll charge 4 melons. I know this isn't exactly hot-button or anything, but it really stresses me out and makes me think I did something wrong. Please fix this.