Mesh modding Tekken 8 causes distortion of the elbows, pending an update to the Fmodel extraction tool.  I devised a workaround for this by modding a lower-body part and pairing it with a vanilla upper-body part that has working arms and neck, namely the short sleeve shirt with everything but the arms and neck masked out.  I am providing this masked version of the short-sleeve shirt, for both male and female characters, as a modder's resource here, which will hopefully be helpful for any modders who want to make mesh mods for Tekken 8 without waiting for a fix to Fmodel.

To use:

1) Make your custom mesh, overwriting any lower-body mesh you want. 
2) Align it's arms perfectly to the arms of the short-sleeve shirt mesh, ideally with a little bit of overlap at the end of the sleeves
3) Remove the entire lower part of your mesh's arms and hands, leaving only that little bit of perfect overlap to the arms of the short-sleeve shirt mesh.
4) Parent to the Armature for the original msl mesh, weight transfer from the msl mesh and export as a msl mesh.
5) TAKE THE SAME EXACT MESH AS IN (4), parent to the Armature for the original prp mesh, weight transfer from the prp mesh and export as a prp mesh.
6) Import into UE, hook up skeletons and material instances, and package as any unused chunk other than chunk #5 (which this mod uses).
7) Install both your mod and this mod in LogicMods
8) In the game, go into character customizer, and select your lower body mesh along with the short sleeve shirt as your upper body mesh.  In the images, I show an example mod (https://www.nexusmods.com/tekken8/mods/33) where the lower body mesh is on the miniskirt with belt (you can stick your mod on any lower body mesh), so when I select any other lower body mesh, you see the masked out floating arms and neck, but when I actually select my custom mesh (the miniskirt with belt), then everything looks good.
9) When releasing on nexus, you can just release your mod and point users to this mod as a requirement.