Just a wild guess what happened, the RO filtering was recently started. When you put low pH water with bicarbonates in buffered soil it can interact with the buffers and actually raise the soil pH. So the pH goes up and suddenly it can't take in iron in that higher pH, and you've got an iron deficiency on your hands. Now that RO water is being used, this shouldn't be an ongoing problem and grower won't need to use "Kentucky windage" when ph'ing water to figure out what you'll get when it flows through the soil because it's now RO and no bicarbonates or minerals to screw things up.
With drinking water, the purification process lowers the pH. So it's not uncommon for municipal water to have bicarbonates to raise the pH to drinking specs (7.2-7.6).
