I agree here, I mentioned that I get this problem here in FL.2. The constant running of the fan and cooling unit is messing with your rH. If you turn off the fan unit and let it sit for 24-48 hours, I'm willing to be dollars to donuts you'll see a major spike in rH. I know thermoelectric coolers are touted as not affecting rH, but in my opinion they do, especially if there's a large difference between the temp setting and the exterior temperature. When the units are running constantly, it does something inside that causes the hygrometers to read a much lower rH. I don't know if the movement of air causes it, or if the condensation occurs in such a way that the wood hold on to the ambient rH, or what, but it happens. I stopped using the cooling fans on both my wineadors because of this, and just try to keep the house a little cooler.
I haven't taken the steps yet, but I plan to see if an ETC will help with this problem.