Librarian Villager Bug

[Aimer]

I've been trying to get a librarian villager to sell Mending books by breaking and replacing their lectern, and eventually the villager offered a Mending book for 30ish emeralds. After I got the emeralds and returned to the villager, the same villager had somehow lost its librarian job, even though nothing had broken the lectern.

Another instance of this weird bug was when I had a librarian selling Protection IV books, and later it somehow changed to selling Sharpness IV books at a different price, even though the lectern wasn't broken or affected at all. Now this same villager is somehow selling Punch II books despite no one touching the lectern.

Does anyone know the fix for this bug?

[Pongmin]

This isn't a bug, I believe there is a chance of a villager changing their trade offers if you never traded with that villager.

Next time trade with that villager when you get the wanted trade as fast as possible to lock its trade

[Aimer]

I see, thanks for the answer!