In Ro or Distilled water the PPm is low (little to no base minerals). Less OH- ions (alkaline buffer) to buffer the acidic fertilizers (H+ ions acidic buffer) so fertilizers will have a greater effect on the ph in Ro vs Tap. My tap water for Example is 150 ppm .5 scale or .3 ec = 120 ppm Ca and 30ppm Mg according to the city lab report. So it has a Higher base buffer and requires more acidic buffer to lower the ph. To combat this tap water buffering like what you see, I feed @ 5.5 ph as it swings ( swing = acidic nutes are used the ppm drops and the ph rises) up into 6.5 + hitting all the micro nutes on the way. I feed daily in coco and mix the feed every time keeping things closer balanced to the ph I chose. Bubbling your water before you mix will help slow the rising ph aswell, chlorine will affect the ph as it dissipates. I use scales and syringes to measure so everything is repeatable and exact. Also you can use water dilution to help with ph aswell. example you could mix up 1L feed with tap water then dilute it .2-.4 Ec with ro. 1.4 1.6 1.2 ec isn't going to make as much of a difference as ph will then dial it in more as you go.
This might not work as well in your water depending on the mineral content. I am in the rocky mountains our water is glacier fed and mountain runoff.