פירוש על שבת 135:7

They teach in the Mishna: to exempt with only one sin offering. An infant who was captured: and who never knew anything about Shabbat. But one who was familiar [with Shabbat] and ended up forgetting: he is like one who forgets that the day is Shabbat while knowing the essence of Shabbat, and is obligated for each and every Shabbat. That it was forcibly forgotten from him: he never knew.

A convert who converted among the Gentiles: before three [a beit din] and they did not inform him of the mitzvah of Shabbat; for if he had only converted by himself he would not be a convert (as they discuss in Yevamot 47b).

Let it teach "one who was familiar and ended up forgetting:" that he is obligated even though one can't declare that during the days in between [one Shabbat and the next] he had awareness to divide [one violation from the next]; and how much more so would one who does have awareness to divide [between one Shabbat and the next] [be obligated for each and every Shabbat he violated].