[Tawātur and Āḥād]

The answer—and Allah is the One who grants success—is that the matter involves detail: namely, that if the ḥadīth concerns a subsidiary (farʿī) issue, then it is permissible that it be mutawātir for the people of the first era and not mutawātir for the people of the second era.
But if it concerns a doctrinal matter with which one is legally charged, and there is no path to knowledge of it except that mutawātir ḥadīth, then it is not permissible for the ḥadīth to become presumptive in later eras.
What indicates this is that if the ḥadīth were to become presumptive in a doctrinal matter, the proof of Allah against His creation in that matter would be nullified. It is known that the proofs of Allah, Exalted is He, are established against His servants; as evidenced by His saying: “So that there may be no argument for people against Allah after the messengers.” [al-Nisāʾ:165].
As for subsidiary matters, if the ḥadīth becomes presumptive after having been mutawātir, the proof remains established by it, and acting upon it is obligatory.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3