Question Scalp shading glitch with direct lighting

[ScrunglyBungus]

I'm experiencing a strange glitch with some hair/scalps where, if a directional light is used, the shadows from that light will not apply to the scalp, giving it a "fullbright" and unshaded look. There doesn't seem to be any rhyme or reason for which scalps are affected by this-- it seems completely random. Is it just me who has experienced this?