Editing Main Render Settings (section)

=== Aggressively Hide Albedo UV Seams ===
Hair combing works by shifting texture sample UV coordinates. For the albedo map, this allows the shader to colour each hair to match the colour at the hair's root.

However, if a sample location is adjacent to a UV island edge, this may cause the sample to be taken from a location that is outside of the visible mesh. If the texture does not have enough "overpainting", then the sample may be black or some other incorrect colour.

The ideal fix is to add more overpainting to the albedo map using Substance Painter, Blender, or whichever program originally created the textures. If this is not possible, then "Agressively Hide Albedo UV Seams" can be used to disable the UV texture shifting for the albedo map. The downside of this feature is that the individual hairs will no longer be consistently coloured, but rather they will change colour along their lengths as the albedo map changes colour below them.