So the devils not vanishing by getting eaten is kinda genius.
Remember, Makima's plan was to eradicate the devils she thought were a detriment to her perfect society.
So having the devils come back is the continuation of part 1. it's a sign that devils are only a representation of human society and emotion and that good and bad things are associated with them. That you can't just erase things you dislike, you have to deal with them and still continue living.
I've read the novel, and continue to read the manga, it starts as cliche OP MC isekai, it also continue the cliche plot of OP Isekai MC with his harem, but at some point the whole story become wholesome and enjoyable to read, even if its outrageous or ilogical, but thats where the charm of this story. 7/10 for me. 2 for the whole story 5 for Nana xD
Remember, Makima's plan was to eradicate the devils she thought were a detriment to her perfect society.
So having the devils come back is the continuation of part 1. it's a sign that devils are only a representation of human society and emotion and that good and bad things are associated with them. That you can't just erase things you dislike, you have to deal with them and still continue living.