The SAME thing happened to me with my Shining O.P. and my Shining R.A. (the blue one with Arms Master Optimus). Was it on the "hip" joint on yours? I think I actually did a mini-review of Shining O.P. & B.2 in the Feedback forum and mentioned it.
Strange thing is I haven't had any stress marks pop up on any other of my clear microns. The Shadow and Hellflame ones, plus Shining B.2, are fine. Go figure. Hopefully the hip joints on the Rainbow Shield R.A.s doesn't suffer the same problem.