Like you, to maintain a good trail knee position, my hips have to make an accommodation they don't really want to make. The easy thing for the body to do is to allow trail knee to (partially) collapse in the backswing.
With lead hand palm down, is your clubface open at the top? As I recall, you once said you felt as though you were pushing down against the ground to begin your downswing--still do that? What part, if any, of your downswing would you say is a dragging action, ie, no overt effort by upper body? Jim