Why would a coolant reservoir be full to the top when the engine is cold?

I’ve got a 2004 Honda Civic DX that has been acting up lately. It started with the inside of the car started blowing cold air when driving at low speeds or in park, and I thought it was maybe the heater core or a head gasket. The temp gauge for the most part was steady, a couple of times it spiked, but instantly fell back down and only driving for maybe 10 minutes. I had the heater core flushed and the rad cap replaced. The temp gauge has been solid since and the issue with the heat seemed to be gone, until recently, then it came back. Now one day I thought I would see if the coolant in the rad was low. The engine was cold (cooled overnight) and there was pressure when I took off the cap and coolant spewed out. Today I looked at the overflow reservoir and it was full to the top of the neck (engine cooled overnight), and no real obvious cabin heat issues. I still think it might be a head gasket, but something doesn’t make sense. Thanks for any comments or suggestions.