I did that test at it was exactly the same.A long shot - check PH of water tank outside the tank. Just pour to a glass and test outside the tank to eliminate that stray voltage and magnetic things.
I also struggled with PH and what I did is to suck fresh air from outside the house to skimmer inlet. That was it in my case and corals appreciated it in a week. Huge difference in a growth. It jumped from 7.8 to 8.2.
Do you consistently feed your skimmer air from outside? How long did it take for you to go from 7.8 to 8.2? If I have to go that route I could probably only do it during daytime hours. Still waiting to see if this CO2 Scrubber will get me any higher.