After certain amount of time, Villagers that have a discount will stop having a discount (or discount becomes smaller).

How to re-create:
1. Open a Minecraft world
2. Go to a village (or spawn in a Villager)
3. Place down a fletching table (to make it a fletcher)
4. Try your best to make it give you a discount (curing, or doing something that gains the player popularity)
5. Wait long enough
6. You may now see the discount either becomes even slighter, or gone entirely.