I often find myself trying to trade with a pig trader as a bird drops a seed or something and they pick it up to eat it, and as a result they stop accepting my trades. Without looking at the code I'm guessing that I'm stopping them from eating the item and they have only a single inventory slot, which is what keeps most traders from accepting more than one trade per day.
Steps to Reproduce
Attempt to give a pig a tradeable item as they pick food off the ground
Attempt to give a pig a tradeable item as they pick food off the ground
A developer has marked this issue as fixed. This means that the issue has been addressed in the current development build and will likely be in the next update.