I could be wrong, but I think that lip shadow is not from AO - I think you're using a directional shadow with not enough blur on your main light.

If you have the figure 90% lit by a single directional light, (i.e. very little or no IBL) then the upper lip will cast a totally black shadow on the lower lip if the light is coming even slightly from above.

To be sure, why not disconnect the AO node on the lips and render again. I bet you still see black in there.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)