Margot Fonteyn was known for having bad feet. I don't know whether this was just a statement about her ability to point (which wasn't good) or because she had foot problems. Watch the standing leg and foot in the 1st partnered ronde jamb to arabesque and near the end there is a series of supported sissones; when they change direction. Some of the problem could have been the partnering, of course. But watch the bourees at the end, and specifically the angle of point of her right foot. But it was that quality of movement, as you say, that was so astounding and for someone with no such good feet, remarkable balance!
Thank you for the post!