The bug

Villagers forget all previous curing history if they are unloaded while they are a zombie or while being cured. If any other players had discounts due to curing they will lose their discounts too. The player curing the villager will still receive a discount but it will act as if the villager was just cured for the first time.

How to reproduce

1. Spawn a villager.
2. Trade or name the villager so it does not despawn.
3. Zombify the villager and cure it once.
4. Repeat step 3 but now unload the villager while it is a zombie.

Once the villager is cured, its prices will not lower. The villager must stay loaded the whole time while it is a zombie (and while being cured) for its cure history to update properly.