Skipping mitzvos regarding Torah learning

Yeshivos sometimes start learning Maseches Kesubos from Perek Eilu Na’aros (the third chapter) or Maseches Bava Basra from Perek Chezkas Habatim (the third chapter). Why isn’t there a concern for the prohibition against skipping mitzvos, since when they open the Gemara they arrive at the first chapter?

Answers from: Rav Chaim Kanievsky shlit”a, Rav Avigdor Nebenzahl shlit”a, Rav Yitzchak Zilberstein shlit”a and many others.

Igud Halacha Challenge Sivan 5778