It could be that it's getting cooler at night,& causing a negative pressure in the FV,so the vodka gets sucked back. As the other fella said,mine doesn't evaporate much at all. You must have some really dry air in your house. Check the DE-humidifier to see if it's set too high. Moist air carries more heat & cold than dry air,using less energy to do so.
Everything works if ya let it-Roady(meatloaf)