Sims 4: Fixing hair before Importing sims to Blender

Hi, Elias . The person in the comments explained it correctly.
Unfortunately, this is how the mesh is modeled by its creator. There is no ability to dictate whether the mesh will be one- or "double"-sided because it is not the sims ripper that adds the other side.

To clear any doubts, I'll explain why exactly this happens and why it was modeled this way.

When you create a plane, it has only one side, facing one direction. On the other side it appears invisible. In Blender you can disable Backface culling to display the other side as well. In TS4 it is impossible to do this with CAS items.

Think of each strand of this hair as a plane. They display textures only on one side but need to do it on both.
Impossible to make them double-sided in ts4, so the only solution is to duplicate them and flip directions.
Doing this doubles the polycount. Polycount is the number of units (triangles) the hair is made of. Higher polycount = more strain on the rendering engine = worse game performance.
To reduce the polycount, the author of the hair decimated the duplicated strands. This led to the change in their shape. The underside started to poke through the front in places where it differs in shape.

Red is where they poke out, purple is where they overlap.

As a result of this, in Cycles those faces that poke out, lets call it, "cast shadows" on the front strands. This causes no problems in-game since cas items do not cast or receive shadow.

So, the hair was made to be used in ts4 and to put less strain on the rendering engine. How the hair might behave in cycles was not a concern of the author and, frankly, should not be. They make hair for ts4 specifically, not every other purpose.

As for the hair length and whether it has anything to do with this issue.
Well, not exactly. Again, this happens because the author decimated the underlying strands. If they did not do this, this would not have happened, even though the hair is long.
Since they did this, longer strands = less straight shape of them = more place for clipping to happen.

To fix this issue, you need to delete the underlying strands that poke out.
You may think "if blender can tell them apart and show them to me in red, it certainly can select them automatically". Well! Not to my or google's knowledge.
- In edit mode enable Face orientation;
- switch to face select and select one red face on a few strands ( a single face on each strand);
- press Ctrl L to select the whole strands;
- Delete - Faces;
- right away you will see the effect. Then just continue with the rest until there's no more left
It is not exactly a fun thing to do, but it will take about 2 minutes

Is this problem or the solution relevant to you, as an aspiring content creator? No, not really, unless you plan on, well, stealing hair meshes from other creators or converting them from other games ( which is sort of the same thing, since they generally do not allow it). This is the only situation i can think of where you might find this useful. For making previews for your creations, you just keep a one-sided version of the hair.