Question

Peace be upon you,
I read in the book Standing Among Those Who Walk about mechanisms for change in Jewish law.
The book brought many sources from the medieval authorities (Rishonim) presenting various mechanisms, mainly those based on the claim that “when the reason has ceased, the enactment has ceased,” but I did not see any discussion of authoritative rabbinic sources on the matter.
In the Babylonian Talmud we have the discussions about “a matter established by a formal count requires another formal count to permit it,” and there are no explicit discussions beyond that. But there are several sources in the Jerusalem Talmud (which were cited by many medieval authorities (Rishonim) in practical Jewish law) in which it is explicit that even if the underlying matter has ceased, the decree has not ceased (Jerusalem Talmud, Sabbath 6:2; Ketubot 1:5; Terumot 11:5; Sabbath 2:1; and also in Shevi’it 4:2 and Sanhedrin 3:5, though in the passage in those latter two sources there is an omission, and the passages require textual treatment).
If so, how can one use leniencies based on “the reason has ceased, the enactment has ceased” if this contradicts explicit, authoritative rabbinic sources?