Not sure if it is system related or not. However, I have NOT had an issue on either one of my systems (a launch 60GB and a 80GB MGS4 Bundle) - both have backwards compatibility, but I doubt if that matters either. I am getting replays/flashbacks in all my games so far, regardless of I use relievers or not, or skip a few innings, or sim a game in between.
I am about to sim forward to May 8 in my Tigers franchise, to see if I have the same issue as the other guy with the Tigers' franchise. Anyway, for me, there's been no issues post-patch.