Update by Jarl-Penguin
Summary
In Java Edition, cured zombie villagers have a certain type of gossip (major_positive) that does not reduce over time and therefore grants the player a permanent discount, but in Bedrock Edition, popularity is not separated into groups, so the discounts will reduce over time and (eventually) disappear.

How to reproduce:

1. Spawn a villager.
2. Turn him into a zombie villager.
3. Cure the zombie villager.
4. Give it a profession and a bed.
5. Check that he has discounts on his items.
6. Wait for a few days.
   -> The villager's discounts will drop to a point where they completely disappear

Original Description
When you cure a zombie villager, the discounts don't last forever. The more you trade with him the more his prices will get. This is normal, but eventually when you trade a lot with them their prices will not go down and will eventually reset back to the original prices. On the wiki page for Java is says that the discounts are permanent. So far this has only been affecting Librarians and Farmers from what I've noticed with the villagers I have.