About two weeks ago my CYA reading was 70 to 80 just like it has been since April. Last weekend it suddenly was 40 to 50. We have the Taylor K-2006 kit. No other parameters are affected.
This seems like a very sudden change even taking into account the test is somewhat subjective. I did try the bucket test to ensure there are no new leaks (we do have auto fill).
Searching TFP I found a few references to unexplained CYA loss such as this one.
We have spent quite a bit of time recently cleaning the tile using pool tile soap purchased at Home Depot. Ingredients are not listed. We also used Mr. Clean Magic Eraser.
Is it possible the tile soap/Magic Eraser is involved?