Nah, you can still get Septem's icon, no need for hard resets, it's just that the way to get his icon is by getting high affection with him.
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.