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I would contact Frigidaire and ask if that is normal. I wouldn't think it would run if the humidity is lower than the setting. https://www.frigidaire.com/Home-Comfort/Dehumidifiers/FFAD3033R1/There's contact numbers under the "customer support" tab, with an email option.
Generally, the dial on a dehumidifier is not calibrated, and simply means allow higher or lower humidity when you move the dial. If things are too dry, just turn the setting down. It may take a few days and adjustments to get it where you want it, and when the weather changes, you may want to adjust it again. It sounds like it is working as designed.
I have the same unit and works well for us. It takes a lot of water from inside our MH.
My Frigidaire Dehumidifier will cycle the fan on for short periods (but not the compressor) if the humidity is lower than but close to the set value. I assumed this is to circulate the air to get a truer read on the room humidity level. Drove me nuts too until I realized what was going on.I use a digital humidistat to monitor the room. As spencerpj mentioned the humidistat on the unit may not be high in absolute accuracy but it will likely be repeatable. I don't think it's uncommon to have to set it 10% or so off from the level you really want to maintain.Tom