The chlorine is high. That's why it took more DPD powder before the color stayed. High chlorine levels well above 10 ppm can bleach out DPD. Your chlorine level may be well above 20 ppm. You should set your SWCG off to 0% output and if your pool is exposed to sunlight then that should lower the chlorine level over time. If you want to lower the chlorine level faster, then you can add a chlorine neutralizer. You can use a 5 ml sample size for your FAS-DPD test so that every drop is 1 ppm FC (saves on reagent).
When the water is colder, chlorine usage drops, especially if the pool is no longer exposed to sunlight.
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