Please hear me out, since I believe that the issue is not caused by lack of uv mapping.

So, apparently I like to suffer, and went on an experiment to reduce the poly manually from a ts4 hair, so after that I could convert it for ts3.

What I did: After getting rid of the undersides, I went selecting strand by strand and hiding the rest of the hair, converted everything to quads, and then dissolving edges by hand.

I just finished it, converted everything back to tris, did the adjustments to ts3 sim head, and exported my mesh to .obj so I could do the undersides on milkshape (I know, I know, but I still can't set blender to show me the transparency of hair texture as it should, I still think it's better to set the undersides on the right order in that godforsaken program).

When I assigned the texture, this transparency glitch showed, I just thought it was some milkshape quirk and went on with my conversion.
In TSRW the same happened, I also thought it was a TSRW quirk.
But oh well, that thing is also in game!

Of course I thought that, in my poly reducing process, it ended up messing the UV, but nope, I took the .simgeom from my package and the UV is how it should be.

So what the hell is causing this?

(also I'm currently traveling, so I might take long to answer questions '-')

Could you please attach the package? I have a few ideas, but no way to know for sure without actually being able to look at it

Looks a bit like a bad underside, I really would not be using Milkshape for this (not just because the tool is redundant, but because the old MS steps for making these were full of misinformation).
Not sure if you've seen the update to my Blender tutorial, but the steps for transparency preview are in the quick reference near the end.

Oh I forgot to say but I also tried to export the mesh right on blender as .simgeom just like your old tutorial, fixed the mtnf and everything, and this was the same result

The package is attached here btw, but it still has some placeholder textures

The rest of the tutorial is in dire need of update next, so at times I may contradict it with newer information.

And thanks!
It looks like a mesh order issue, where faces that should be underneath are showing on top. It can be previewed by using the Alpha Blend blend mode.
I was able to fix it by removing the undersides, then selecting the entire mesh and using Separate > By Loose Parts. Blender is quite smart at separating them in the right order, so just scroll to the end of the list, select the last one, then scroll to the top and shift-click the first. Right-click the viewport and click Join.

Recreating the underside in Blender goes as so:
Duplicate the object (select in the Scene Collection, Ctrl C then Ctrl V), flip the normals of the duplicate, then click the original and ctrl-click the duplicate, move your mouse into the viewport and right-click > Join.
With Face Orientation on, the mesh should be purple. Purple = blue faces are on top, magenta = red faces are on top. I vaguely remember the old tut saying either are fine, which you can shoot me out of a clown cannon for.
Same concept as the first fix- Blender will merge them in the order they're selected.

It wasn't the main culprit, but the downscaled Milkshape underside can also cause visual issues which is why I recommend against it- this is the difference between the two:


Side note, I had to convert your mesh to .obj for it to not crash Blender when I tabbed into Edit Mode. Not sure what has kicked the bucket there, so you may want to copy fresh GEOM data from reference.
To save you some time, I've since learned you just need to save the .geom in SimGeomEditor without the mtnf swap

OMG thank you so much! I really had no idea I could reorder the strands like that (and without going to the nasty milkshape!)
Seriously, this will make dealing with hair so easier!

Also I really get confused everytime with the shift+click and ctrl+click to select stuff in the right order...

Anyways, it got fixed! I just need now to fix the bones and work on the textures ^^

Oh, me too. I even had to edit my reply because I got them backwards myself
Blender really doesn't make it easy to tell the difference between the active selection being one shade of orange lighter. But it's at least not too much of a hassle to redo it in the opposite order.