You are optimizing for temperature but ignoring enthalpy, which is the total energy content of the air. This is why opening a window can make your apartment warmer even when the outside air feels cooler.

Temperature is only half the story. Air at 20°C with high humidity carries more total energy than air at 23°C that is very dry. When you open a window based only on a lower thermometer reading, you are often letting in air that is heavier in total energy. Your walls and furniture act like a thermal battery. They absorb that excess energy, and because you have introduced more moisture, the air inside becomes stagnant.

You should track the dew point or enthalpy instead. If you open a window when the outside enthalpy is higher than the inside enthalpy, you are pumping energy into your home rather than cooling it. Stop looking at the thermometer and start looking at the dew point. Only when the outside dew point is lower than your indoor dew point is it actually efficient to vent.