The Gemara says (in Yevamos 42b-43a as well as other places) that "וכי רבי לא שנאה ר' חייא מנין לו". In context of Yevamos, it means that if there is an argument in a Mishna and a Stam in a Braisa, we don't follow the Stam Braisa since how did Rabbi Chiya know that the Halacha follows that opinion if his teacher (Rebbi) didn't.
Who was R' Chiya?
If he was a Tanna, he'd have the authority to argue on his teacher (as we see Rebbi and Rabbi Shimon Ben Gamliel arguing). Why do we "ignore" his Hilchasa K'Ploni?
Even if was an Amora, we bring proofs from earlier Amorayim to support claims of earlier Amorayim. Why is he worse than Rav, Shmuel, or Rav Yochanan on this?