This is true if your kicking a banana bag or a human body but usually when your kicking a heavybag the circumference of it is a lot bigger than a body so even a perfectly placed kick will have some foot contact and make a pop instead of a thud. The bag I usually kick is so big that if I kick it to make a thud per say, I would have knee contact lol
Ha ha, I'm from the U.K we haven't progressed to kicking heavy bags yet, we still practice on the weekends by kicking idiots out of nightclubs. But yes it's my bad, I spoke about kicking thai pads but you are right about where the foot meets the shin bone if your kicking the heavy bag.
Overall by far the best way to improve technique is to join a Muay thai gym, you'll learn from more experienced guys and learn correct technique, and get to see how it works in sparring if you like that.