Shifting Clicking the trades will show the all traded items xp value's being put into the xp bar but it only put as one item's xp value into xp bar when you right click the villager again.
MC-147012
Shift Clicking Villager Trading XP Bug
Resolved
Duplicate
4
Basically, the preview of the xp bar and also the trade locking act as if you did the max number of available trades but only one trade is completed.