So... after 5 years, I finally finished my project of recoloring every hairstyle... just to discover that it causes some weird graphic glitch in life mode, where the sims wear their own skin as a overlay.
My mods that cause this are my mermaid, vampire, and alien eyes, fish tails, and some of the hair recolors: snowy escape, the updated afro hairs, all SPs, and holiday celebration.

And yes, I own the game since release and never actually played it, don't judge...

Is this a problem with the cc item, or is it the expansion itself? Can it be fixed?
Does anyone else have the same problem, and if you do, is it the same items/packs that cause it?

Impossible to say with no information. Please post pictures of the problem with an explanation of how it's supposed to look if it's not obvious, and at least one of your packages that has this problem.

sorry - I didn't know how to post pictures and package files.

It looks normal in CAS mode, but in life mode... well, you can see it.

I think this is a mipmap problem. Your eye textures have no mipmaps, and while that should affect only the eyes my guess is that it's affecting the rest of the sim for reasons known only to EA.

Anyway, try either saving your textures as DDS with the option to generate mipmaps checked and import them again, or save them as PNG and import them that way.

Oh nooo....
I think I figured it out - I got a new computer in the middle of doing this stuff, and I when I installed my drawing program, I must have forgotten to check the mipmaps stuff in the saving settings, that's why only half of my cc doesn't work. That's not that bad, because I still have all of the files, but a lot of work - sorry for the inconvenience. I tried changing one of the packages and it seems to work.