The farmers no longer appear to trade for carrots, wheat or potatoes. The first trade for a farmer is Pumpkins. I searched for this in case it was deliberate but could not find mention of it on any forums. Apologies if this is a new feature rather than a bug.
MC-140745
Farmers trades are missing
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Looks like the first tier trades are missing for each type. cartographers start at compass and shepherds at selling wool.