Villager trades revert to regular rates despite being converted from being a zombie villager.

I had a librarian that gave me a discounted trade of 18 paper for 1 emerald then I tried to lock out it's trade. After 2 in-game days (3 resupply instances), the villagers trade did noy revert back to 18 paper to 1 emerald and instead permanently increased its trade to 23 paper for 1 emerald. I waited yet 2 more in-game days (4 resupply instances) and still not bringing it back to 18 paper to 1 emerald trade. It's frustrating and a waste of golden apples which I can't farm at the moment since I'm still at the early game stage.