There is air entering the system somewhere. I would pressurize the system with the spark plugs out. With pressure on the system, use a borescope to look into the cylinders. You may see the leak in there. A coolant leak in a cylinder will knock the carbon off and make it look clean/newer than the rest. So note that as well while doing a borescope. It may be external as well. Use some windex or something on the top of the tank where there's no coolant. The windex will bubble where there's air leak. You might not find any leaks either. With the engine cold, it might be fine. When it's hot, it changes the growth of the metal/gaskets, and will leak then. I've pressure tested cylinder heads and are fine when cold. Wen they get to 180°, then they opened a crack. This is what I would do. Take it or leave it.
Like mentioned, new parts aren't always good parts, so don't rely on that to fix a problem. Maybe something wasn't installed correctly? Maybe something has been damaged during all the overheating the engine has encountered lately?