Villager's XP bar fills up more than it should when pressing Shift for one item trades

If I trade a villager 7 emeralds for a Leather Tunic which it will only give me one (trades like this), if I press Shift to get the Leather Tunic, the XP bar will fill up more than it should and when I exit and enter the trading UI, the villager XP bar goes to where it should be, please fix.