Hard lead does tend to lead a bore, at lower velocities and pressures. Allows gas cutting. You need a really hard "bump" to fill the bore and grooves with harder lead. In modern guns, the lead being too hard will often lead a barrel very badly. (and yes, visa-versa with very hot loads, then you NEED the hardest lead possible)
This could hold true in a muzzle loader, makes sense that you want soft pure lead to "bump up" easily. And of course there are other variables involved, comparing modern guns and ML's. My point being, harder lead can cause leading, even though it seems counter-intuitive.
Truth, a barrel will vibrate differently, clean or fouled. So there's always going to be a POI difference. But, that can be minimized, even to the point where it can hardly be detected. Certainly for hunting, to where it makes no difference.