Edit: I read too fast and didn't see that you and SchwarzwälderTorte already discussed this very issue (reading is hard). :'D
I'll keep the post up though as an extra reminder for the devs, hoping they'll pick up on this particular point of feedback because I agree the missing line is a big problem.
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Looking at those comparisons something that I was never really able to put into words suddenly became clear to me.
(I can't edit any images right now. I hope I can explain it well enough.)
In addition to all the criticism people have pointed out, 6.x mouths have a much deeper shadow around the line where the upper and lower lip meet, leading to a darker "line" between the upper and the lower lip.
So even if the shape itself may be similar (e.g. both versions have the slight "downwards dip" of the upper lip in the middle of the line where the upper and lower lip meet) the visual effect is completely different due to the weak, almost non-existent, shadow between the upper and the lower lip.
This strong shadow adds a lot of character and quite literally defines the shape and the expression much better, resulting in a more striking and pronounced appearance.
This is a really important element for the unique look of the different mouths because even though the line between the lips itself may technically be similar if you look really hard you just don't properly perceive it if the defining shadows are lacking, robbing it of its characteristic impact.
The 7.0+ lips look so wishy-washy in comparison because the line between the upper and the lower lip is so weak and undefined that it just doesn't stand out as a feature and has no strong character.
This issue is not related to the lighting engine I think because this line's shadow is always weak and undefined.
This is why it's not enough to fix the outer shapes of the lower and upper lip (e.g., cupid bow), the expression (slight smile or frown) and the duckface/puffy cheeks (although all of them also need to be fixed!)
They also really need to add this definition between the lower and upper lip back in (e.g., by "hand-painting" the depth back in).
(Sorry for the formatting. Trying to grab the devs' attention without images.)