SOLVED!!!
After 6 months, I think I've finally found the problem to the grey mouth & eyelash glitch. It's a combination of the last Maxi eyeshadow (silvery white one after the cateye) and a mapping issue with various skintones. All of the shadows do it to a certain degree, this one was just more noticeable.
Removing it fixes the sim, but there's also a fix for it, so that the shadows are still usable: MTS - Sclera correctors that change iris less, and also fix gray eyelashes and gray mouth
So simple, yet sims have died over this!