Villagers change trades when not supposed to

I've built a trading hall where the villagers have access to their jobsite block. However, whenever one villager's jobblock for the first time in hte minecraft day, the other villagers start flashing quickly from their job texture (in my case a librarian) to the unemployed texture, and when they are done, they go back to their job, but have changed their trades. This is very annoying because everytime I obtain a Mending book, the trade swaps and I've lost it again.