When Noctis gets sucked into the crystal the crystal erases Noctis from the memory of everyone who has ever met him or known including that of his lover Ignis until Noctis comes back to Eos in which everyone's memory of him is restored. Which creates some problematic issues once this happens.
+Established relationship spanning since the brotherhood era
+Ignis having not remembered Noctis got into a relationship with someone else (an oc not one of the bros or another known character) and once he remembers Noctis he feels horribly guilty because he has basically been cheating on Noctis for ten years
+Ignis, Gladio and Prompto without their memories of Noctis became virtual strangers to each other and when they get their memories back they have to re-work their relationship to each other
+Noctis stays a week or two with them in Lestallum before heading to Insomia
Noctis/Ignis- Crystal memory loss
+Established relationship spanning since the brotherhood era
+Ignis having not remembered Noctis got into a relationship with someone else (an oc not one of the bros or another known character) and once he remembers Noctis he feels horribly guilty because he has basically been cheating on Noctis for ten years
+Ignis, Gladio and Prompto without their memories of Noctis became virtual strangers to each other and when they get their memories back they have to re-work their relationship to each other
+Noctis stays a week or two with them in Lestallum before heading to Insomia