Farmer plants then immediately harvests crops

I built a large 27x27 automatic carrot farm divided in half with one farmer working each half.  The farm was working just fine until I added a dirt ceiling right above the farmer's heads, leaving a 2-high gap for the farm.  My intention is to create additional farms above the carrot farm.  Once I enclosed the farmers with dirt they both started acting erratically by planting, then immediately harvesting the planted crops staying within probably a 3x3 patch of ground.

I tried a few different things to see how reproducible it was.  I determined that the farmer tended to get stuck in two locations diagonal from the center drop off point.  Creating a skylight at these locations (open air, transparent block, even farmland) caused the farmer to go back to normal operation.  A solid block causes the ai to start behaving erratically again.  Ceiling height did not appear to affect the issue, as I cleared out a 3-high ceiling and it still had the same issues.