I have potential heard that the mefresh on nedarim is not Rashi. If so, who is the author? Is there a clear source that it is not Rashi?
There is a brief discussion about this in the beginning of Metivta edition of massechet Nedarim, I'll summarize it here:
The commentary on Nedarim which is referred to as "Rashi commentary" was first printed in 1521 in Veneza in the first ever Talmud print (see wikipedia for Talmud Bomberg).
The first person to hold that the Nedarim commentary was not from Rashi was Rabenu Betzalel Ashkenazi (author of Shitat Mekubetzet), as written by his student Rabenu Shlomo Adeni, author of the book "Melechet Shlomo".
The Chida (Haim Yosef David Azulai) also argues in his book "Shem HaGuedolim" that Nedarim commentary is not actually from Rashi and also proves it from the difference in the language used by this commentary and other Rashi commentaries in other massechtot. There he points out that also the Beit Yossef when referring to this commentary refers it as only "the commenter", which he wouldn't do when refering to Rashi. Also the Yaabetz (Rabbi Jacob Emden) and Maharatz Hayot (Rabbi Tzvi Hirsch Chajes) both write that the commentary is not from Rashi.
Rabi Tzadok HaKohen from Lublin in his responsa Tiferet Tzvi (part 2, 21:3) writes that the commentary in massechet Nedarim is from Rabenu Guershom or at least was taken from his commentary [fun fact: from pages 22b to 25b the commentary is for sure from Rabenu Guershom, as is written explicitly in the Rashi section].
Also, some rishonim indeed quote Rashi in massechet Nedarim, that suggests that Rashi indeed wrote some commentary for Nedarim and, in fact, some parts of Rashi commentary in massechet Nedarim were copied by Rabenu Betzalel Ashkenazi in his notes of massechet Nedarim and on his book Shitat Mekubetzet.