CYA is by far the toughest test to run.

Do you:
1) shake the mix of pool water and reagent for at least 30 seconds?
2) Stand facing the sun?
3) Hold the measuring tube at waste level and SLOWLY add the mix till the dot disappears?
4) Dump the mix back into the vial and run the test a second time with the same mix?
5) Perform the test the same way every time?

If you do this, then your measure is accurate and theirs is not to be relied on.

I speculate that your CYA only got to 30 recently and you've been battling chlorine breakdown until then.

I would shock the pool to 15ppm and keep it there for a couple of days, and see if it mostly holds. If it keeps dropping, especially overnight, you are still fighting something and need to maintain shock levels until then.

Carl