possible reasons:

+3ds multipart models dont store the origin relative as you save it from your modeling tool, but they will place the model related to the origin into MED. if your model origin is placed somewhere outside of the mesh this can cause placements all over the editor. Check if the importet triangle size matches the exported ones!!

+uvw map...3ds doesnt require mappings to be exported right. med could just refuse to load them proper without a mapping.

+animations and 3rd party tools methods... maybe some of your actions or settings in your mesh cant be read or exported proper as a 3ds file.

+your 3dgs version needs to be able to read 3ds files

+flipped normals...check the faces of your arms if they could be flipped (=the normals of the tris face inside the body instead outside)

just some reasons i could think of

cheers and hope it helps