Book villagers change their trades to despite having traded with them before

In this last version I realized something, the book villagers that I only traded with once to secure their trade had suddenly changed their trades again despite having already traded with them before, it's a bit frustrating since searching for enchanted books is something tedious for having to be removing and putting the latril until something you are looking for comes out, I do not know if it has to do with the fact that they do not sleep in their beds, I have those villagers in an iron farm and the beds are inaccessible for them, I play on nintendo switch, I don't know if this happens on other platforms, if it happens to someone too, please comment to be aware.