(edit) I have fixed this hair by extracting the GEOMs and running the entire thing through DABOOBs as if it were new, including new scalp and face textures. The latter originally pointed to EA keys that looked fine on inspection, but I figure that was the source of the grey smear.

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Hello, I'm usually more of a lurk and figure things out through trial and error kind of user but I'm a bit at wits end with a piece of custom content behaving unusually.

I've been retexturing the hairs of a creator who hasn't been around in the community for about a decade.
When testing ingame, the hair paints a grey shadow on the Sim's face. I figured this was error on my part and booted up with the old file and found that the original does the exact same thing.
To rule out a dodgy file, I even found an existing mesh-included retexture and discovered that it too had the exact same problem.

My version pictured above.

Simply being broken CC is one thing, but I simply cannot wrap my head around how the original AND the retexture had screenshot examples with no sign of the issue. Why would a retexture of a broken hair exist to begin with? It has to be something on my end, but I keep a very meticulous game with maxed out settings in all departments so the graphical errors I was familiar with in the days of playing on a laptop are long behind me.

I cleared caches between every test, tested on different Sims, and tested on a completely CC free new save.
The effect doesn't at all look like a shadow effect being cast, it's static and applies to the head like makeup.
It is present ingame outside of CAS, and it even shows on the CAS thumbnail.
I have even tested removing the scalp texture, with no change.
Over hundreds of hairs, including a dozen or so by the same creator, I've never seen anything else do this.

Full disclaimer, I did not create this hair.
This is RoseSims3's Hair 021. I'd gladly be working backwards if my own work had resulted in this error.
Has anyone encountered this before and know which part of the hair is specifically doing this?
If images exist of the hair being used without this error, is it possible the hair is so old it has issues on high graphics settings that aren't visible on lower ones?

Thank you for your time.