The lack of valid early evidence for the Assumption does not matter: it is what Rome "remembers" that makes it binding doctrine.
Before Mary's bodily Assumption into heaven was defined, all theological faculties in the world were consulted for their opinion. Our teachers' answer was emphatically negative . What here became evident was the one-sidedness, not only of the historical, but of the historicist method in theology. “Tradition” was identified with what could be proved on the basis of texts. Altaner, the patrologist from Wurzburg…had proven in a scientifically persuasive manner that the doctrine of Mary’s bodily Assumption into heaven was unknown before the 5C; this doctrine, therefore, he argued, could not belong to the “apostolic tradition. And this was his conclusion, which my teachers at Munich shared .
>But,
subsequent “remembering” (cf. Jn 16:4, for instance) can come to recognize what it has not caught sight of previously [because the needed evidence was absent] and was already handed down in the original Word” [via invisible, amorphous oral tradition] - J. Ratzinger, Milestones (Ignatius, n.d.), 58-59 .
“the mere fact that the Church teaches the doctrine of the Assumption as definitely true is a guarantee that it is true.” — Karl Keating, Catholicism and Fundamentalism (San Francisco: Ignatius, 1988), p. 275.