That does seem like a lot even for direct sun at 40 CYA. What is your pool temp?
I would bump your FC up targeting 6 or so tonight. Test after it's circulated for 30 minutes, then test early tomorrow morning. Overnight chlorine lost test (OCLT). You should not lose more than 1 ppm overnight. If you pass that, I'd go up to 50 CYA and see if that helps.
You are letting the FC get too low ... you need to stay ABOVE the minimum at all times. See the FC/CYA Chart. Now that the FC has dropped too low, something could certainly be growing in the water making the FC drop more.
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