Trading with a farmer villager for Potatoes, on the second trade the Potatoes in the trading window disappear and you get no emeralds.

First I traded potatoes/Melons/Pumpkins and Beetroot with him until all were locked then after opening the trade window again, some of the trades were unlocked and this happened.

see video:

https://www.youtube.com/watch?v=hNO1ntlVv7Q&feature=youtu.be

After playing around with it for a bit it happened with other trades as well. Not sure when it triggers but I lost some books, emeralds, melons and potatoes.