Villager can make the discount number of unstackable item more than 1

In 19w14a there's a new villager trade menu. After clicking a trade it moves all required items you have to the trade item slots and you actually can make a trade even if there's a invalid discount.
You can even make item stack of (for example) 6 boats, as I wrote here: MC-147431.

Also, MC-147431 is not a duplicate of this issue as MC-147431 is about making nonstackable items stack even if there's no invalid discount.