Thank you for your reply.
I have taken another series of pictures with a different background and lighting. If you look at the forehead now, you can see it clearly.
Okay, I can see it now.
When you post process these, are you doing it on the figure by itself, then adding that layer to a set background?
Because when you apply filters (such as the Camera Raw set of filters in Photoshop, as an example) ... sometimes the filters have a very different outcome, depending on the 'size' or 'amount of pixels' they have to work with.... it's hard to explain, but for example, you apply a filter to just a foreground figure on a transparent background. The effect off that filter will change a little bit if it's on a background. I think it's because filters run their algorithms by examining all the surrounding pixels and then determine the final effects to apply.
I'm explaining it poorly, but it means that if you took this figure in one frame and applied a filter that bumped the exposure up 0.25 for example... the effect would be different if you applied that to just the figure on a transparent BG, over if you layered the image on the BG first and then applied that effect to the full frame.
So during an animation series, the amount of pixels for the filters to use for it's calculations changes because you model turns her head and what not. For the most consistent results across hundreds of frames, you should assemble your parts and then apply any filters you want to the whole frame renders.
I don't know what your normal process is, but I usually render the series as normal. I then drop all those frames into my denoiser. Then I do any touch ups in photoshop. Finally, I create a custom Action in Photoshop to first assemble each frame by adding the animation render to the background, and then apply the filter I want to that completed frame.
I see you have a reflection in that mirror behind her... so I was wondering if you are actually rendering the whole series as complete with the background?
In either case, my gut feeling is that effect under the hair has something to do with some post-processing that is being done after you render the frames. It's also possible that this is just a strange artifact from DAZ and that particular hair asset. There's probably some animation filters out there that could normalize that effect. Or maybe you can try running this series through DAIN and seeing if that normalizes it some?
My animations are still pretty crude, so I am working on that more than anything right now. I'd love to have this being the problem for me, lol!