Not in a huge amount of detail I am afraid. It certainly does seem that some of the red colours in that image produce output colours through the RRT + Rec.709 ODT where the green and blue channels are both zero, resulting in the “clumping” you describe.
I colour picked the ACEScg values of a random pixel in the clumpy area using Nuke, and got the values [0.08774, 0.00324, 0.00047]. That gives chromaticity coordinates of [0.692021, 0.307992] which is right on (or maybe just outside) the spectral locus.
You may want to send the image as a sample to Graeme Nattress at RED, so he can examine it.