Im pretty sure you can only get Septem’s icon back if you hard reset the game. Ive gone back through Septem’s route a few times and you don’t regain the icon, as you get it one during the first route and thats it. I did Septem then nulla, which is probably why I can’t get septem’s back haha
Ik, ive already gotten it. you cant get the icon twice, and Nulla permanently removes the icon if you do his route AFTER septems. Nulla gets jealous that we have septem's icon and removes it in place of his. I have already gotten septems icon, it just permanently gets removed by Nulla if you do his route after such.
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.