You're in peat based soil I bet.
Keep the soil pH at 6.5-6.8 and if that means watering with a slightly higher nute solution, so be it.
Now, when you take the water pH, are you doing it before or after you mix the nute solution? 'Pure' RO water with a high pH is indicative of an aging filter. The pH you are concerned with is that of the solution including all nutes and additives you are using.
I'd try to adjust your nute solution to no higher than 7.2, and still keep checking runoff.