This is still true of a sub-dial at 9 as well.It is a matter of geometry and the seconds subdial's critical 6 - 12 axis. When at the 6 position, the minute hand will be parallel to that axis occasionally, but regularly, blocking the second hand from view. With the subdial at 9, the hands can never be parallel to the 6 -12. Of course, some of the subdial will be covered with the minute hand, and you will "lose" the second hand when the minute hand covers the 3 - 9 axis, but that is not as important given that most people want to know the exact minute vice, 15 seconds prior to the minute.
All of this may be a reason why two versions of the famous Unitas handwound movement were created -- 6497 & 6498.
Good question, heb