VOL,

I don't know of a cause to lose CYA in an opened, swimmable pool.
(Winter...yes)

I would chalk it up to "one of those things" and go back to 40ppm. I'll bet you stay there all Summer.

Say, were you chit-chatting on that new, waterproof phone and having an adult beverage when you did that 25ppm test? Yep....that explains it!