Can a minhag trump halacha?

Rosh haShana 15b  (I copied from Sefaria with the translation below) --  the upshot of the gemara is that even though though the minhag to maaser carobs based on the harvest year contradicts what the din should be, since we are dealing only with an issur derabbanan, the minhag stands.  

(In this case the minhag was based on a legitimate minority view.  Sometimes what people call a "minhag" is based on nothing and therefore may not carry the same weight.)

אָמַר רַבִּי יוֹחָנָן נָהֲגוּ הָעָם בֶּחָרוּבִין כְּרַבִּי נְחֶמְיָה 

Rabbi Yoḥanan said: The people were accustomed to act with regard to carobs in accordance with the opinion of Rabbi Neḥemya, that their tithe year follows the time of the fruit’s picking.

אֵיתִיבֵיהּ רֵישׁ לָקִישׁ לְרַבִּי יוֹחָנָן בְּנוֹת שׁוּחַ שְׁבִיעִית שֶׁלָּהֶן שְׁנִיָּה מִפְּנֵי שֶׁעוֹשׂוֹת לִשְׁלֹשׁ הַשָּׁנִים 

Reish Lakish raised an objection to the opinion of Rabbi Yoḥanan from a mishna that teaches: In the case of white fig trees, the Sabbatical Year for them with regard to the halakhot of eating and elimination is in the second year of the Sabbatical cycle, due to the fact that their fruit grows for three years, and so the fruit that ripens in the second year of the Sabbatical cycle had already taken form in the previous Sabbatical Year. This indicates that the tithe follows the time of the formation of the fruit and not the time of picking.

אִישְׁתִּיק אֲמַר לֵיהּ רַבִּי אַבָּא הַכֹּהֵן לְרַבִּי יוֹסֵי הַכֹּהֵן אַמַּאי אִישְׁתִּיק לֵימָא לֵיהּ אָמֵינָא לָךְ אֲנָא רַבִּי נְחֶמְיָה וְאַתְּ אָמְרַתְּ לִי רַבָּנַן

Rabbi Yoḥanan was silent and did not respond, as though he had no answer. Rabbi Abba the priest said to Rabbi Yosei the priest: Why was Rabbi Yoḥanan silent? He should have said to Reish Lakish as follows: I am speaking to you of the opinion of Rabbi Neḥemya, and you say to me the opinion of the Rabbis?

מִשּׁוּם דַּאֲמַר לֵיהּ שָׁבְקַתְּ רַבָּנַן וְעָבְדַתְּ כְּרַבִּי נְחֶמְיָה 

Rabbi Yosei the priest answered: He could not have made this argument, because Reish Lakish would then have said to him: Do you abandon the opinion of the Rabbis, who constitute the majority, and act in accordance with the opinion of Rabbi Neḥemya, who express a sole dissenting opinion?

וְלֵימָא לֵיהּ קָאָמֵינָא לָךְ נָהֲגוּ וְאַתְּ אָמְרַתְּ לִי אִיסּוּרָא דַּאֲמַר לֵיהּ בִּמְקוֹם אִיסּוּרָא כִּי נָהֲגוּ שָׁבְקִינַן לְהוּ 

Rabbi Abba the priest asked further: Rabbi Yoḥanan should have said to him: I am speaking to you only about how the people practice and that their custom follows the opinion of Rabbi Neḥemya, and you say to me that it is a prohibition? Rabbi Yosei the priest answered: He could not have said this, because Reish Lakish would then have said to him: Where there is a prohibition, even if they were accustomed to act in a particular manner, would we leave them to continue?

וְלֵימָא לֵיהּ כִּי אָמֵינָא לָךְ אֲנָא מַעֲשֵׂר חָרוּבִין דְּרַבָּנַן וְאַתְּ אָמְרַתְּ לִי שְׁבִיעִית דְּאוֹרָיְיתָא

Rabbi Abba the priest asked further: Rabbi Yoḥanan should have said to Reish Lakish as follows: I am speaking to you about the tithe of carobs, which is only by rabbinic decree, as by Torah law all fruits apart from grapes and olives are exempt from tithing, and you speak to me about the Sabbatical Year, which is by Torah law? This being an irrefutable argument, the Gemara once again clarifies this matter.