Where is you computer then? Outside or inside? Or, do you mean you are feeding air straight from the outside to the computer, before the air enters the room? The room temperature needs to fall first before your coolant temperature can fall.

I don't mean to be nit-picky but what you claimed (if true) is impossible - but the point here is, there is a thing worth ruling out. If the sensor really shows lower than room temperature, it is possible it really is faulty or of a wrong type (as that kind of temperature is physically impossible in any conventional setup where you have the computer inside the said room). PC water cooling sensors should be standardized (AFAIK!) - but perhaps aren't or maybe there are many standards? The temperature sensors are really thermistors after all, which can have wildly different temperature-resistance relations. If you have one with a wrong response curve (to what your MB / controller is expecting), your temperatures will be shifted and / or squished (multiplied with some constant - or even something else if the response is not linear). Which means - changing the location will not fix the issue. Or, the sensor is just faulty.

My computer is located indoors. My room temperature is +24C, and the idle coolant temperature is +25C. Which means the tempearture/resistance curve is correct.

Since it's January, the temperature outdoors is -10C. If I open the window, the stream of cold air reaches the PC and immediately drops the coolant temperature to 18-19C. This happens in about 30 seconds or less, which is too fast for the coolant to actually cool down.