While trading multiple trades (shift key) with Farmers buying potatoes, they will only allow one trade regardless of a full stack of potatoes. Waiting for refresh does not change status. Will still trade only a single gem for whatever amount of potatoes are required in that round. Also, if I still keep trading with potatoes in the same round of trading, the potato offer will eventually close ('X'), but all gems I earned after the first one are taken away and my potatoes are returned to my inventory!

Villagers will do multiple trades with carrots fine. Not sure if this happens with any other items in Villager trading tables.

I have tested this on two different farmers who buy potatoes, and it happens consistently with both.