Interesting findings sir. My pH is at 6.3 after I mix in the powder but then of course like you said I use some pH down after. I'm guessing it really is the bicarbonates neutralizing the acids. I got my tap water tested at a lab for brewing beer some years ago and it could have obviously changed by now but it had pretty high carbonate/bicarbonate levels.
So maybe that 50ppm from my R/O is mostly carbonates/bicarbonates since at 50ppm starting water both our Ph's sit at 6.3 after mixing. I wonder if your 8ppm starting water without calmag or potassium bicarb can still get a starting pH of 5.9 with stability after mixing. hmmmmmmmm.