Meanwhile, AI has also pointed out to me
"The exact column texture: tree-level Yukawa universality in heterotic Z_3 x Z_3 orbifolds" (Navid Ardakanian)
which claims (section 8.3) that up-quark and down-quark yukawa matrices in this class of models are, in a first approximation, left-circulant. (If they are exactly left-circulant, there is no CKM mixing, so there must be corrections, possible sources of which are also itemized in the paper.)
I do not recall ever before seeing a string paper with circulant yukawas so this grabs my attention. Of course originally we want this for the charged leptons, and in these models the leptons arise differently to the quarks (section 2, second paragraph). Nonetheless, approximately-circulant yukawas for up and down quarks are my preferred explanation for the waterfall (#291 in this thread), and
@arivero 's new paper reports a good "inverse-Koide" formula for the downquarks which also implies circulant yukawas (#321, #324). So this surely deserves some kind of study.
However: the author is an independent researcher with his own genAI business and no visible history as a physicist prior to this year (he has four papers on arxiv now), who lists Claude in the acknowledgments. So one might want to be on guard, even more than usual, as to whether AI is telling us what we want to hear, rather than the truth.