[Solved] Lightning/Texture Issue?

Hi! Upon export ALL meshes get their normals a little messed up here and there. This happens every time you export a mesh so you should fix it manually on your own before exporting (the best way to see how your mesh will look like is changing the texture to solid as you did in your image and pick "smooth shade" from the sidebar in object mode). Sometimes it is noticeable in Sims 4 Studio but because the object is textured is difficult to see them.

Normals basically determine how the light is cast, certain row of vertices get duplicated upon export, so you should blend them in again so they are smooth and not sharp. This is why we use the "remove doubles" tools. I'm quoting another reply that explains how to fix this. Instead of selecting the whole mesh what you should do is do it by "strands". Select a whole strand including the separated squares and "remove doubles". Then follow to the next strand. The inverse of this, which is to make the edges hard again (for example, the last row of a top or sleeves) is "edge split" For this you select edges (with the edge selection mode instead of vertice selection - second button left to the circled one in the image- to select the exact row of edges you want to duplicate and create a hard edge).

If you need any specific help please share the blend file.