Thanks for the post.
Your CYA was very likely zero. There’s no mystery to where CYA goes - it oxidizes from exposure to chlorine at a rate of about 5-10ppm per month depending on water temperature and UV exposure. During the hot months in the desert here, I’ve lost as much as 20ppm in a month. The second route is biological - there are algae and bacteria that can use CYA as a source of energy. CYA is consumed and converted to ammonia and then, if you are lucky, a different set of bacteria will convert the ammonia to nitrates and nitrogen gas. TFP gets plenty reports every year of winterized pools that are opened only to have a huge FC demand caused by ammonia formation. Once that happens, it’s often easier and cheaper to simply drain as much of the pool as possible, refill and SLAM.