the fact that this doesn't happen on all characters you have should be a big red flag for you. take one that renders without seams, and plug in the texture maps for one that renders with seams, and save that as a new material set. see if it renders well or not. skin shader setups are so complex you'll probably never diagnose what exactly is causing this, if I were you I'd just fix it.

you might also contact the artist(s) that made the problematic character(s).

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