Even though Nephi doesn’t give a full genealogy of his fathers, we learn of his tribe in the House of Israel: "For it sufficeth me to say that we are descendants of Joseph" (v. 2). Later on, readers learn that Lehi was specifically "a descendant of Manasseh," who was one of Joseph’s sons (Alma 10:3). It is possible that Lehi, based on his lineage, was a descendent of refugees from the Northern Kingdom of Israel (where Manasseh’s tribal grounds were situated). When the Assyrians invaded Israel around 732–722 BC, Lehi’s ancestors likely fled to Judah and settled in a precinct of Jerusalem called the "Mishneh." Familiarity with this historical backdrop can help explain why Lehi was living in Jerusalem around 600 BC, instead of in the tribal grounds allotted to Lehi’s ancestor Manasseh. As refugees they may not have been able to bring records with them, even if they had once had them. They may have had a general knowledge of their northern ancestry through oral tradition, but may have been unable to prove this or know it in full detail running all the way back to Joseph, about ten centuries earlier. After Lehi and his sons obtained the plates of brass, they had proof of what they had previously only believed from oral tradition, hence Lehi then "knew that he was a descendant of Joseph" through Manasseh and could prove his important status as such (1 Nephi 6:2; Alma 10:3).
Jeffrey R. Chadwick, "Lehi’s House at Jerusalem and the Land of His Inheritance," in Glimpses of Lehi’s Jerusalem, ed. John W. Welch, David Rolph Seely and Jo Ann H. Seely (Provo, UT: FARMS, 2004), 81–130.