What does it mean when a hair has transparency issues?

No.
In ts4 the only way to enable CAS transparency is through Simglass shader which is meant for lenses. Lenses do no overlay each other, there is only one plane. Transparency issue happens when one plane is in front of another an both of them have simglass shader assigned. The game is not coded to render the mesh in the right way, it cannot tell which mesh is the closest to the camera and should be rendered first because you are not meant to be using it for hair. So, to deal with it you need to tell the game how to render it.

The reason 3ds max is said to be a better software is because it can replicate the same (it is called sorting) issue within the software while compatible versions of blender cannot. Newer versions can but I do not know how efficient they are (I’ve seen a pic and it looked like a mess).
However, I’ve also seen someone replicate said issue in the compatible version but no one else seem to be able to.

Also, no, there is a modifier that can add the other side of the planes automatically. It’s called solidify and you need to disable Fill rim option

Read everything explained in this thread for more info regarding the logic behind layering and how to do it in blender
sims4studio.com/thread/3857/most-transparency-issues-alpha-glass


also planes are the worst (most time consuming and troublesome ) way of making hair, I suggest to look for tutorials that are not related to TS4. NURBs, hair system + cloth dynamics, sculpting + retopology are better ways but you need to really look into them  

Thanks, Mauvemorn. This is an example of a hair that has a top plane with a plane underneath clipping through. It's made by Skysims. So, proper sorting is what's needed to fix this? It's doesn't matter if there's clipping? 

And thanks for letting me know about the solidify modifier--seems my understanding of how the planes were made is way off mark. 

NURBS. I hate working with NURBS, ugh. 

Transparency is a common problem when setting up hair in Sims 4. However, it is much better than the constant problems with textures observed in previous parts. I think it is worth working out the drawing and graphics of each strand in more detail. Thus, improving the quality and eliminating this problem will be possible. In any case, the characters in the game never lose their hair for biological reasons. Although nowadays, there are clinics such as www.novahairtransplantnyc.com/, which solve this problem with a small surgical intervention. What is your experience in