Shift-clicking a villager trade then shift-clicking a second trade converts the item from the second trade to the items in the first trade

shift-clicking the trade of a villager (buy all until can't make a trade), will cause that while doing it another time on a different trade causes the secondly purchased items to become the first purchased item.

Steps to Reproduce:
1. find a villager that have two trades where you give him emeralds
2. give yourself a nice stack of emeralds
3. shift-click the first trade
4. shift-click the second trade
what should happen is that the items from the second trade will magically become the items in the first trade.

images are provided.

NOTE: the images are 19w13a. however, i checked and it works in 19w14a (b still not checked)