I'm glad everything came back okay!
Maybe you've already ruled this out, but "just in case:" was this by any chance while you had someone staying in your home? Perhaps that person had something, e.g. soap, shampoo, scent from laundry detergent that might have triggered something?
I don't remember how long the person was there, though . . .
Or was it maybe while you're neighbor was having the pool removed?
I think Pookie hit the nail on the head when she suggested Mazy's problem was lack of acid rather than too much. I think the ACV is helping her a LOT.
This is just a theory based on what I've learned about digestion, but I'm going to toss it out there: stomach acid digests protein (meat) and helps break down the minerals in the food so they can be absorbed. It does
not break down carbohydrate. Sooo . . . . what if . . . by eating carbs (kibble), the stomach was "trained" over time to produce less acid? Then someone changes the diet to one with more animal protein, BUT the stomach isn't getting the signal, for whatever reason, that it needs to produce more acid? This was my thinking when I suggested the ACV for Mazy, with the hope that, over time, the stomach would be "retrained" to produce more acid on it's own.
So going with my theory, I wonder if some cats that were fed kibble for significant periods of time, don't produce enough stomach acid and need something like ACV added to their food in order to improve digestion and also to "retrain" the stomach. Mind you, I have no proof of this and haven't found anything online about it. It's just a theory.
I would continue to keep an eye on Arrows (and I know you will) and document as much as you can, including any environmental changes that you can think of. Keep us posted, and we'll keep trying to help.