Thanks @hanatos for the updated lut and sharing the GitHub issue!
I replaced the old one.
Indeed, just looking at the coefficients maps, they look way smoother past the spectral locus.
I compared the old LUT and the new one on the same flower photo (that I exported to linear ProPhoto RGB).
(left) new LUT - (right) old LUT
Indeed as you predicted the effect was not very visible.
But that’s still great, probably I could relax a bit the regularization.