Was it a theme intended for the note 7 (or at least for some of the "-edge" Samsung devices)?
Yup. I changed themes and happened to me.
If you change the theme back does it reappear?
It seems to be happening to several users. I would think it might be a good idea that the affected ones write to Samsung reporting the bug. They might fix it in the next ROM update...
Maybe a 'Clear Cache' & 'Clear Data' of the Edge apps tray can fix this once & for all. I have not used an 'edge' phone (using Note 4), but generally such 'disappearing apps' issues can be fixed by above action.