2006 Chevrolet Impala Coolant issue

Replaced original radiator cap appx. 6 months ago. Original had deteriorated o-ring seal.
Since then, the coolant level in overflow tank NEVER changes when hot or cold. Prior, the coolant level in overflow changed very slightly.
Curious to determine whether the replacement cap (Stant purchased at O'Riley's) may not allow hot coolant to escape into the overflow. Should coolant level in overflow increase every time car is driven and reaches normal operating temp? If not, why not?
Car warms up as usual and heats normally.