In the early stages of a blown head gasket the pressure in the cylinder push around the sealing ring in the gasket through very narrow slits. Remember the cylinder pressure can reach 1800 psi. During the intake stroke the sealing ring would only be exposed to 18 inch of vacuum. That 18” may not be enough to draw coolant into the cylinder or at least enough coolant to hurt anything. It s pretty common for the first symptom of a failing head gasket to be pressure in the coolant system and spitting coolant out the overflow. Once the head gasket get worse and the sealing ring opens up the system can no longer hold pressure and the pressure issue can go away. That is when the car will starts eating coolant.

One thing I did assume is that the original poster knows that it is normal for coolant to get pushed into the overflow until the car is warmed up and then the coolant should be drawn back into the radiator as the car cools down.