On V4's and M4's, the white line is precisely on the limit between the face material ("1_SkinFace") and the ear+head ("2_SkinHHead").
I have seen such weird effect once, on a Vic4, near the nose flesh and I had to change the parameters to get around this.

From what I could understand, it's as if some parts are intersecting with one another. Maybe try giving another scattergroup id to one part, to see if that forces Poser to avoid that not desired intersection by setting the order in which the skin parts are computed.

𝒫𝒽𝓎𝓁

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