I cooked a decent size pork roast last night between 400-450(alternating between direct and indirect side of my BGE), and it had a pronounced smoke ring. Even more so than on some of my longer cooks. This is not the first or only time I've had that happen either. It happens when I cook pork tenderloins all the time as well.
I don't profess to know all the chemistry, but it would appear that temperature alone is not the determining variable.