I have to agree with you on this one. I've had to help students (including my wife) fix this exact error (different art, same problems, sometimes). Somehow, she learned the arm mechanics and failed to put the proper power generation with them. When I became her instructor, we started working on making it a full punch. She still struggles with the habit. When punching down, for instance (what I call a "drop punch" because you can simply drop body weight into it), she still tends to keep her body stable and punch downwards. Because she starts at an appropriate distance for a correct punch, she is often too far away for this incomplete punch. It's taking real effort for her to correct this ingrained mistake.The issue is if you are used to isolate your arm from your body, you will develop bad habit and you will need time to remove that bad habit later on.
IMO, just by using "turning stance" is far from enough. You will need:
- rotate your body,
- bend your leg and then straight your leg.
I don't know how WC instructors avoid building this bad habit if they start by teaching "arm only", beyond a single training session. (For some people, it is easiest to learn the limb movement without the body, but I only use that for the very first session on a given strike.) I assume they must have some tool in their bag that helps make the transition.