I had my pool nicely balanced, with CYA at 50. I normally keep my FC at 2-3, with Borates. I then did an AA stain treatment, including 3 bottles of Jack's Blue Stuff as sequestrant. The Blue Stuff got up to around 18 ppm.
A couple of weeks later, when I tested CH, CYA and Borates, my CYA was barely registering on my TF test kit. (Followed by a thankfully minor algae breakout.) Other than a bit of rain and a couple of backwashes, there wasn't any significant dilution of the pool. The pool did turn milky after the AA treatment, which is normal, but filtering and clarifier took care of it.
The only other unusual factor is that I changed the zeolite in my filter after the AA stain treatment. (It was due to be changed anyway, so I figured I'd be certain to get rid of all the metals that were lifted, sequestered and filtered.)
I'm bringing the CYA back up, but I'm curious if it was the AA and/or Jack's Blue Stuff that disappeared the CYA and, if so, why. Is there anything else that can eat CYA? Is there any chance the reagent for the CYA test suddenly went bad and the test results are incorrect? (It was fresh this spring.)