To be honest, there wasn’t much science behind it rather than plotting the chromaticities of several offending values across a variety of problematic shots, then adjusting the primaries such that the problematic chromaticities fell into a more favorable portion of the rendering primaries.
As described here, this matrix just moves the effective rendering primaries.
Most of the problematic images that I designed with came from Alexa, but this issue exists with other cameras too. I suspect that your problematic values are falling outside of my effective rendering primary space. I’d be able to confirm this with actual values from your image that I could plot. Again, image context isn’t important to me, I just need to be able to calculate the chromaticities. When you get a chance, please send me at least one [R,G,B] triplet from the problematic area to better investigate.