I really have no idea what the programmers did to fix this bug in the "coming sometime" patch, but fix it they did. I did do some testing on it ages ago and it was now working correctly, giving the results I expected, but I can't find where I put my results.
The official description of the bug fix is The "spilling" of large harvests is fixed. This will allow the full effects of irrigation and the iron age modifier to finally be seen. Production for non-irrigated farms drops very slightly in this release (though typically only 100 units).I really don't know how much effort is worth putting into testing farms yields before the patch comes out. I have no information on when its coming though, its all in the hands of the Sierra localisation people.