The Gemora says that a student who parts from his Rebbi must bid farewell again if he remained within the city. This law is brought in the Rif, Rosh and Shulchan Aruch (Y.D. 242,16). Why does the Rambam ignote this Halachah?
This Halachah is mentioned by the REMA, but not by the Shulchan Aruch, nor by the Tur or Rambam, who mention only the manner in which a Talmid should take leave of his Rebbi (i.e. not turn his back to him, etc.).
It seems that their understanding of the Gemara was that Rebbi Shimon bar Yochai did not hold of that Halachah (and when his Talmidim said, "But didn't our Rebbi teach us...," they were either saying that out of respect, but he did not actually teach that to them, or indeed he had taught it to them but had rescinded his view). Hence, the Rambam is ruling like Rebbi Shimon bar Yochai, that it is not necessary to take leave of one's Rebbi a second time.