this tiferet yisrael is definitely not ‘simple pshat’ in a mishna.
It goes straight against the rama , shach and accepted halahik practice.
Rama permits/ mandates to be holek in halaha on your rebbi [even rebbi muvhak – rov torato mimenu] if you have ra’yot.
Others even if it is only miSvara.
YD242:3
Kol hameharhar ahar rabo [sanhedrin 110a] comes from the pasuk where yehudim falsely ascribed to Moshe rabenu [who provided ‘man from heaven’ as food for the dor hamidbar], that this ‘man’ has the non-existent attribute of blowing up inside one’s stomach. [rashi on humash]
In other words , making up non existing baloney as a negative for your rebbi, that falls under kol hameharhar ahar rabo.
That the rebbi made a mistake , that s not included in kol hameharhar , and is definitely possible, and is permitted to be said by the talmid. as per rama above.
Kedai to keep in mind , however, that like the rebbi is liable to make a mistake , the talmid is even more so [kal vahomer]