After testing for an evening, I found that the Back-focus problem is:
- more severe near the 70mm, starts to disappear at 135mm.
- more severe with longer focussing distance.
- not related to which AF point being usedĄD

I've got a temporary solution for reference:
~Back-Focus Solution~
a) Focus at 200mm first (can also apply AE lock in advance).
b) Lock the focus by half-pressing the shutter button.
c) Change to the original focal range.