I discovered that reducing root softness does not wholly solve my issue (although it's a bit improvement). I take your point, carodan, on camera angles, but I don't think that's the issue in this particular case, since I've been rendering different materials with exactly the same camera position. The blonde materials are rendering far better than the dark ones.

There's not much difference in the shaders, though, other than colour. I do have a Spots node plugged into a ColorRamp to give some color variation, and I have differing Spot Sizes. I wouldn't think that would have anything to do with this, would it?

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Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3