4 Nephi 1:20

and he kept it eighty and four years, and there was still peace in the land, save it were a small part of the people, which had revolted from the church, and took upon them the name of Lamanites; therefore there began to be Lamanites again in the land.

He kept the record for 84 years. There was still peace in the land. But a small group of people left the church. They called themselves Lamanites. So there were Lamanites in the land again.

Amos kept the record for 84 years. Peace still reigned, though a small group broke away from the church and started calling themselves Lamanites. So the old division crept back into the land.