What a thread! I also know the pain of sourcing bleach in Yakima! Last time I was there the Nob Hill stored 10% indoors, but had older stock while the greenway walmart stored it shaded outdoors and had fresher stock! Anyways, I ended up with the ultra fresh and indoors 8.25% from both fred and wal, whichever I was closer to when I needed it. The Nob Hill WM ended up stocking fresh 10% on the last days of my slam, better late than never.
While not an official TFP observation, I've seen higher FC demands following backwash in some pools. While this could be due to the backwash itself, it is likely due to adding the replacement water which, if city, has chloramines in it, which counts as CC, which do burn some FC to oxidize before the sun gets them. Additionally, the lower FC of the fill water must be thoroughly circulated for the sample to be representative of the average FC of the pool's current water. In most pools with good circulation, 30 mins, but some pools require longer, potentially up to 2 hrs. If well/reclaimed/river/irrigation water is used, then FC demand can be sky high after additions.
Your FC loss was a little less steady that we're accustomed to seeing, but not unprecedented. By any chance are you aware of anyone ever using or did you see any containers for green out/yellow out/swamp treat/green fix/non-chlorine shock/anything with bromine/bromide? Not important, just curious.
Because your CYA is potentially 80+, I suggest you do step #8 when you get a chance here
https://www.troublefreepool.com/content/220-CYA
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also see the picture of the test's endpoint linked in the lower section to confirm you're stopping at the correct look