In Hoshido, the last unpromoted character you get during the main story is a level 14 Nishiki in Chapter 12.
In Nohr, the last unpromoted character you get during the main story is a level 15 Flannel in Chapter 14.
In Invisible Kingdom, you get Elise, Effie, Harold, and Charlotte ranging from levels 7-10 in Chapter 14, Nyx at level 9 in Chapter 15, Asama and Setsuna at levels 13 and 11 in Chapter 16, and Odin, Zero, Pieri, and Lazward ranging from levels 12-16 in Chapter 17. Given that Chapter 17 is the last one to feature any unpromoted enemies, that all seems like a bit of an issue.