Villigar trades randomise when I log back into a world

This has happened so many times that I don’t even remember any specific examples. But when I save and quit a world and close the game after it’s done, and play again later on or the next day, my villigar trading hall will replace some of the trades they have with new ones! I normally only get librarian villigars for enchants, and have noticed this happening the most when I’m breaking and replacing their work stations rapidly to make them give me books I actually want. That’s all I have on the issue, other than several hours wasted on villigar breeding and trading.