The same thing happened to me a while ago and the only thing I could figure out is they add some stuff to the water, I believe they are called buffers. And the buffers make the PH rise back to it's original PH. If you google "buffers in water" it says this. "A buffered solution is one that resists a change in its pH when hydrogen ions (H+) or hydroxide ions (OH-) are added. Water that is not buffered is subject to drastic changes in pH by addition of an acid or base. "......... I don't PH my water anymore after I had this PH problem. And I have never had any problems, as long as my EC is on point I am fine. I done a shit load of experiments on different types of water and when you add 1 drop of PH down to RO water it drastically changes the PH and stayed the same, from my guess work. I guess you could add calmag to RO water and then water down some PH up/down and create a perfect PH solution of water/feed to give to your plants but I've never dived in that far as I can't find much info on it, my tap water is around 6.5/7.0 and by the time I add nutrients it drops to around 6.2/6.3