The use of RO/DI is not causing your pH to drop.
Not having the minerals and carbonates in the tap water that you once used, is allowing the tank to do what it normally will do when you just add *water* back to the tank for top offs.
The water only evaporates, not the minerals and carbonates. They remain in the tank - but natural biochemical process tend to cause the pH to drop over time in most all our tanks.
Tap water is usually pretty hard, and now that you have ceased adding these additional carbonates and minerals like calcium/magensium, that you were adding along with the tapwater - your tank water would be expected to drop pH somewhat - if you are not supplementing with either a pH buffer, alkalinity builder or kalkwasser.
Shadow678 give excellent advise - start a routine of dripping kalk and all will be as it should again. Dosing the tank with marine buffers or alkalinity builders is another option as well.