And according to this, it explains the difference between the approach of the Rambam and the approach of the Rash, because according to the Rash, since the reason is a part of the fulfillment of the mitzvah of matzah, if the reason is not present, then the mitzvah of matzah is not fulfilled, and one should go back and eat with the reason. But according to the Rambam, the mitzvah of matzah stands on its own, and eating a kezayit of matzah with the reason is a mitzvah on its own, so if one eats a kezayit of matzah without the reason, the mitzvah of eating matzah is still fulfilled and one has already fulfilled his obligation, but he has missed out on the mitzvah of the reason, so according to his approach, there is no need to go back and eat with the reason.

And it seems that the Rashbam holds the same as the Rash, as is precise from his wording: 'Matzah requires the reason, when one eats matzah of the mitzvah on the first night,' implying that the reason is attributed to the act of eating the matzah itself, and not to the overall meal.