Librarians have a book and quill trade, where you can trade 2 book and quills for 1 emerald. By default, this trade is impossible since book and quills cannot stack. The game appears to have some kind of system to prevent this, so the trade is always discounted to one book and quill for one emerald regardless of the player's village reputation.

In Bedrock Edition Beta 1.18.10.26, this trade was changed to use 2 slots of the trading interface, fixing the problem.