oh, really? I've gone through his route a couple times after Nulla's and its never been replaced...
Are you perchance skipping through the dialogue or changed a bit some of your choices? Because I think that may be the issue on why, try to do the same choices you did when you first got his icon, also I did their routes again just to make sure so i went to septem's to nulla's to doing septem's once again and yes he replaces the icon.