When you cure a zombie villager they are deeply grateful and give you excellent discounts on all their trades. However, they seem to forget after a software upgrade, after (approximately) Minecraft 1.14.2.

I noticed that all the villagers I had converted from zombie villagers in my trading hall lost all their discounts when I upgraded my (survival) game from 1.14.2 to 1.14.3-pre2.

However, newly-converted zombie villagers still give a discount in 1.14.3-pre3, so I assume the 'grateful non-dead' discount is still intended to be given, and is still intended to be permanent.

I believe the versions of the game affected are all the 1.14.3 pre-releases, though this issue may go back further, it may affect 1.14.1 and 1.14.2.

EDIT: This is still an issue in 1.16.4. I found in my latest game in 1.16.4 (NO upgrades involved) two zombie villagers stopped being grateful after a short time, with no obvious reason why.

STEPS TO REPRODUCE:

1. Create a world under Minecraft 1.14.1-2.
2. Spawn a Zombie Villager, and cure it (Potion of Weakness/Golden Apple).
3. Assign a profession to the healed Villager (any).
4. Observe substantial discounts on items offered for trade.
5. Log out, and upgrade the world to 1.14.3-pre[x] (any x).
6. Check the Villager's trades again. No discounts are offered.
7. Spawn a new Zombie Villager and heal it as before.
8. Assign a profession to the new Villager.
9. Observe the new healed Villager has discounted trades.