Trading with villagers in C and S mode, items in the product box sometimes won’t allow shift click, or it will, but it disappears and/or gives you back the emerald/item you used to trade with, and the GUI will drop a full stack of emeralds when you exit.

I can confirm this, although (again) I'm not a mod.

Duplicate of MC-1580 , please use the search function to see if your bug has already been submitted. Currently over 35% of tickets are being closed as duplicate.