Why do we get negative effects instead of barter discounts with a villager zombification setup?
After further investigation, testing and asking around, I found out that there is nothing wrong with the setup.
The problem was that I was playing on minehut and I had no idea that they add a paper.yml setting "fix-curing-zombie-villager-discount-exploit" that by default is set to "true". This is to fix the supposed "bug" that we wanted to "exploit" (Mojang does not seem to have confirmed nor denied that is a bug).
For those that say that we might have hit/killed a villager: no, the NBT data is clear, the minor/major negatives are targeted towards the zombies, not us.