3 Nephi 1:30

and thus were the Lamanites afflicted also, and began to decrease as to their faith and righteousness, because of the wickedness of the rising generation.

Thus the Lamanites were also afflicted, and began to decrease in their faith and righteousness because of the wickedness of the rising generation.

So the Lamanites suffered too. Their faith and righteousness began declining because of the wickedness spreading through the younger generation.