In the book "Sefer Yosef Breslevi", there's an essay by Yisrael Ben-Shem in which he raises multiple questions related to various appearances of people from the Tribe of Binyamin throughout Tanach. One of the questions he asks is about the phrase "שבטי בנימין" (the tribes of Binyamin) which appears twice in Tanach, once in Shoftim 20:12:
"וַיִּשְׁלְחוּ שִׁבְטֵי יִשְׂרָאֵל אֲנָשִׁים בְּכָל שִׁבְטֵי בִנְיָמִן לֵאמֹר מָה הָרָעָה הַזֹּאת אֲשֶׁר נִהְיְתָה בָּכֶם."
And once in Shmuel 1:9:21:
"וַיַּעַן שָׁאוּל וַיֹּאמֶר הֲלוֹא בֶן יְמִינִי אָנֹכִי מִקַּטַנֵּי שִׁבְטֵי יִשְׂרָאֵל וּמִשְׁפַּחְתִּי הַצְּעִרָה מִכָּל מִשְׁפְּחוֹת שִׁבְטֵי בִנְיָמִן וְלָמָּה דִּבַּרְתָּ אֵלַי כַּדָּבָר הַזֶּה."
He writes that only Binyamin is ever referred to as being made up of multiple tribes. The question is, of course, why? What does this mean?
He answers that indeed Binyamin was made up of two distinct ethnic groups which eventually merged: Tribe A: The descendants of Binyamin, natural Israelites; and Tribe B: A group called "Bnei Yamin" or "Bnei Yemini" which, besides being distinct phrases in Tanach, are also seemingly attested to having existed in a tablet discovered in the ancient city-state of Mari. According to him, this answer solves all of the other questions he raised, such as 'why did the people of Yavesh Gilad burn the bodies of Shaul and Yonatan if that wasn't Israelite custom?'
The classic commentators, from what I've seen, explain that the plural term refers to the families or clans (בתי אב/משפחות) of Binyamin - each family or clan within the tribe was also called a "tribe". Ben-Shem points out that this explanation would work, were this a one-time thing or if it was used to describe more than one tribe, but as it's used twice and both times referring to the same tribe - seems odd, to say the least.
Is there a more Orthodox/traditional answer to this problem?