I don’t know, but I’m not surprised given the FC loss. There are a bunch of threads on CYA loss. It happens, rarely, but there are many that have experienced it. At least you know.I don’t even think I’ve backwashed since then! The loss would be from evaporation/refill only. My reagent is not expired and I’ve done this test successfully before.
cya does not evaporate. If you have 1000 gallons, evaporate 50, then add 50, you will have same cya level. Kinda like if you have glass of salt water and let it evaporate, all the salt will still be in the glass. Put water back in, you have salt water.