If you are using an automatic humidity control and heat setting, then yeah...may or may not use much water, depending on the temperature and humidity in the room where you sleep and how long you sleep (longer = more water, obviously). I find it can also depend on the mask being used and its fit....more leaks = more air moving though the humidifier = more water used.
My S9 on auto for temp/humidity did the same thing. I personally like a little more humidity so I keep mine on manual. With my FitLife TFM, I have the heat and humidity down a bit....makes it a bit more comfortable for the face...and the machine sips water as my unintentional leak rate is pretty much 0.
My nasal pillows, which I wear on occasion, get a bit more heat and humidity to keep the nasal passages open. The pillows leak a bit more for me so I'll go through ~1/2 a tank a night, maybe a bit more, over the same amount of sleep time.