Why do we get negative effects instead of barter discounts with a villager zombification setup?

I have the following setup for villager zombiefication/healing:

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It is a sequence of 5 of this:

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Basically the minecarts drag the villager in a "pit" where they get zombieifed, then I push a button and the cart moves to another pit, where I drop a splash potion from a dropper and I give the golden apple to the villager (with right-click). It repeats for 5 times on each side (we are 2 players).

Now, we just finished dealing with our first villager, but to our dismay the discount was not what was expected.

I looked into the NBT data, and instead of finding two major_positive bonuses of 100 each, I found them to be equal to 20 (i.e. one single healing) and several minor_negative and major_negative. Specifically: the major_negative was towards the zombies, not the players.

Is there something wrong with the setup? Is there some rule not written in the wiki on how to get the bonus for healing a villager 5 times?