The cooling system is a vessel. The radiator cap is the portal to that vessel. The cap seals a portion of the vessel -- the expansion tank. As you fill the tank, the fluid either goes into other portions of the vessel -- the radiator and block, for example -- OR it comes out of the vessel. If it is not coming out, then the vessel is more empty than you thought, so keep filling.

You are not having any issues with over-temp, or not telling us of any, so you should check the level each morning before you start the car and top as needed. You should find that after 2 or 3 more cycles, the coolant should stay at the correct level. There is a bleed screw that allows trapped air to escape, but generally this screw only needs to be opened as one of the steps of completing a service call that requires draining of the system to accomplish. The trapped air can be displaced by the coolant over time, and in this case the expansion tank -- the tank under the radiator cap -- will empty, which is the symptom you are reporting.