Yea that's completely correct. kH helps buffer the pH and keep it stable, but if you have active substrate it is doing the buffering so keeping the kH around zero you minimize the "work" the substrate has to put in to lower the pH.
Yea I meant are you sure the kH and gH values from your RO/DI filter are correct? Or is your TDS measurement in degrees? Generally, if you're using a TDS meter it will measure in ppm, whereas the kH and gH tests are in degrees. In which case your TDS should at least be ~38.
Thanks. I guess I should stop adding baking soda now and stop wondering why my KH keeps going back to 0 lol.