It turns out that your pH observation is almost perfectly correct. With the very low alkalinity of RO water (typically under 20 ppm as CaCO3), it doesn't take much acid to cause the pH to drop like a rock. Even the Sparge Acidification Calculator in Bru'n Water confirms the result. Using an alkalinity of 13 ppm, 33L of water and 1ml of 88% lactic is predicted to produce a pH of 4.3.
Sparge water pH is not really the target a brewer should aim for. Alkalinity is the more appropriate target. I recommend bringing sparging water alkalinity to under 25 ppm to avoid astringency problems. With alkalinity as the controlling variable in mashing, it turns out that the final pH of the sparging water that a brewer should use will vary based on the starting alkalinity of the water. In the case of very low alkalinity water like RO water, the pH to achieve an appropriately low alkalinity may be 6. Whereas, a very alkaline water might need to be acidified to bring the pH to somewhere around the low 5's to get the alkalinity low enough. As Neal found, he didn't need to acidify his sparging water at all.
There is only one ion that has extra utility in the mash, calcium. Beside its effect on mash pH (along with Mg), it also precipitates oxalate which forms beerstone. While you can add minerals directly to the kettle to account for not adding minerals to the sparging water, I suggest that providing a minimum calcium concentration of 40 ppm in the sparging water should help reduce oxalates in the beer. Other than that, I suggest that adding salts to the mashing or sparging water can be optional. Those salts can sometimes be added directly to the kettle to produce a desired water profile. The most important factor is making sure that the mash pH falls into a desirable range. A program like Bru'n Water can help guide a brewer to what and when to add salts to their brew.
Enjoy!
