@Malik I use 3ds max with a free plugin "textool" for the packing and calculed texel density. https://www.scriptspot.com/3ds-max/scripts/textools-toolbox-for-the-texture-artist
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Next thing I would do is duplicate the problem mesh, delete history, do a humanIK quick bind and then export that and test it in engine. If that works correctly, then you know the problem has to do with the mesh itself rather than the import/export process. If this is true, then I'd probably start by copy the current skin…