Bug: When I completed Jaylin's chapter's secret ending I got Flake's Gallery and not Jaylin's, it eventually fixed itself, but still
Ah, thank you for letting me know! This may be something that stays in the game due to how it handles unlocking content with each chapter. If you have a character and their song or image gallery doesn't appear, try swapping through the different girls in the Character Select menu; this will trigger some flags that should unlock the bonus gallery and songs for each girl unlocked.