[Frequently](https://www.sefaria.org/Eruvin.50b.15?vhe=William_Davidson_Edition_-_Aramaic&lang=bi), when the Gemara tries to disprove Rav from a Mishna, it answers that he's [considered](https://www.sefaria.org/Ketubot.8a.15?vhe=Wikisource_Talmud_Bavli&lang=bi&with=all&lang2=en) a Tanna and can argue on a Mishna.

But there's this case in [Berachos 37a](https://www.sefaria.org/Berakhot.37a.9?vhe=Wikisource_Talmud_Bavli&lang=bi&with=all&lang2=en), where Rav and Shmuel both say that one doesn't say a Mezonos on rice, and the Gemara concludes with a "תיובתא" (a conclusive refutation) against them. 

Why doesn't the Gemara answer here that Rav is a Tanna?