On further experimentation, the issue might be related to the process that's lowering the desired trades... For example, when I have a trade offer for 3 2 books -> 1 emerald, initial trading lets me trade at 2:1, but when the trade completes, additional books are deducted from my "offer" when I'm pulling them back to return to my inventory.
Recent example: I had 36 books, trading with a master librarian showing 3 2 books -> 1 emerald. Put all 36 books in the offer, traded for 1 emerald at a time. Placed the 6 emeralds in my inventory when the villager stopped wanting that trade, went to pick up the 24 books still showing in my offer, and as soon as I picked them up, the stack size was reduced to 18.
I tested further with a villager that offered 5 4 emeralds for 3 bookshelves (and 1 emerald for a lantern). Placed exactly 4 emeralds in my offer. When I went to pick up the bookshelf, the villager traded me a lantern instead and took 1 emerald from me.
Also, when I had exactly 16 emeralds offered to the same villager, shift+clicking initially placed 4 bookshelves in my inventory. When I closed the villager trade UI, my inventory was changed to 3 bookshelves and 1 emerald.
So, from what I can tell, the trade UI isn't applying the "discounted" cost that it's displaying, but it winds up recalculating it on close of the UI or when you pick up your offer.
On further experimentation, the issue might be related to the process that's lowering the desired trades... For example, when I have a trade offer for
32 books -> 1 emerald, initial trading lets me trade at 2:1, but when the trade completes, additional books are deducted from my "offer" when I'm pulling them back to return to my inventory.Recent example: I had 36 books, trading with a master librarian showing
32 books -> 1 emerald. Put all 36 books in the offer, traded for 1 emerald at a time. Placed the 6 emeralds in my inventory when the villager stopped wanting that trade, went to pick up the 24 books still showing in my offer, and as soon as I picked them up, the stack size was reduced to 18.I tested further with a villager that offered
54 emeralds for 3 bookshelves (and 1 emerald for a lantern). Placed exactly 4 emeralds in my offer. When I went to pick up the bookshelf, the villager traded me a lantern instead and took 1 emerald from me.Also, when I had exactly 16 emeralds offered to the same villager, shift+clicking initially placed 4 bookshelves in my inventory. When I closed the villager trade UI, my inventory was changed to 3 bookshelves and 1 emerald.
So, from what I can tell, the trade UI isn't applying the "discounted" cost that it's displaying, but it winds up recalculating it on close of the UI or when you pick up your offer.