Maybe there isn't anything wrong with the engine. My car and motorcycle engines won't idle either, until they warm up good.
I have some other glow engines and they aren't all that good for idling until they warm up good too.
But I do have some engines that idle fairly well when you first start up the engines cold.

Hummm.
One other thought is when you first start the engine, the fuel tank is full of fuel, but at the end of the flight the fuel tank is low on fuel. Thus if the engine is idling well at the end of the flight, but not at the beginning, then maybe it is running too rich at the beginning. Id the carb is set up well, then it starts to look more like a fuel tank location issue in relation to the carburator. You could try only putting in a small amount of fuel and start the engine cold and see how it behaves with a low fuel level inside the fuel tank.

But then I see this a lot on other engines as well. I even have a couple that do that as well. But with a ARF plane you tend to be limited in how good of a fuel tank location you can get.