Not quite clear, what you are trying to say. Does this mean it shows the intended cost centre in the posting run log, but is using a different one in actual posting? I wouldn't believe that.
Of course: if we are in a retro scenario, the system will akways have to address the old cost centre as well to reverse that old posting.
I'm also not clear, whether you've done the fixed cost assignment. Here I understan what you write as yes. Further down someone making the same suggestion and you don't seem to know what to do in that view (basically: if you don't find it intuitive, the IMG docu explains it well