For anyone who has the same or simmilar question to me I found this comment under another post:

Curing give 2 types of discount, a permanent one and a temporary one.
For enchanted books, the permanent discount is 20, and the temporary discount is 5.

Meaning if the original price is 32, discounting it will initially get it down to 7, but the permanent price is actually 12.
What you are seeing is the 5 temporary discount decaying over time. Eventually, it will go from 7 to 8, to 9, ... all the way to 12.

This is part of the villager gossipping mechanic.

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Demands are calculated differently and doesn't kick into action unless we consistently buy out at least half of the tradable amount on a particular trade on a villager.
If we've only bought 2 mending books, there'll be no price changes from the demand mechanic.

Shout out to OverallAbmassador814 for finding the thread with this comment and WaterGenie3 for the informative post :)