I don't know if it explains anything or not, but fwiw, I just noticed the other day while doing some bodyslide work that this "tanline" appears to very interestingly enough follow the same lines as the collisions mask
the_seam.jpg 150.39KB
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PS> When I closely examine my own female's loverly derrière, I also see some evidence of this line, so I don't think it's just you.