Don’t have any idea why it is going so low… I’ve never used those, BUT… If you would rather “get something” in your “pH UP” adjustment rather than just adding NaOH, et al… consider adding either SiO2 or K2O3Si… Not sure about the Silicon Dioxide (because it is largely insoluble at room temp), but Potassium Silicate is pretty alkaline and will help get your pH back up in range while providing some silica for your plants.
Note… for K2O3Si, you get 18.204 ppm Si and 50.685 ppm K out of a 100 ppm dilution. I’d mix the Potassium Silicate as a separate mixture and get a ppm reading… then multiply the ppm reading/100 by the two numbers I gave earlier to get the ppm that would be added to your nute mix IF you add the entire gallon. IF you only add a part of the gallon to the mix just til the pH is “right”, you would multiply your gallon ppm totals for Si and K by the portion of a gallon you used ( X ml / 3785 ml).
Typically, you add Si first, but at such a low pH, you can probably add it as a pH adjuster with little to no precipitation.
BIG NOTE: Be SURE that you are calculating the additional “K” you are adding and that it doesn’t run you too high on “K”.
Just a thought…