I found three ways to reproduce this:

1. Select a trade that requires emeralds, and then use quick trade on another one. When you close the UI or select any trade, You'll lose your emeralds.
2. Select a random trade, and then use quick trade on any option. You won't get the items.
3. Select a trade that doesn't require emeralds, and then use quick trade on another one. When you close the UI or select any trade, your items will be dropped.

Workaround:
Don't use quick trade until it's fixed. Take every item manually.