Regarding editing existing body part meshes

[Krenman]

Im new to modding body parts and couldnt find the answer to this anywhere so i figured i'd ask here, blender keeps breaking the mesh ingame after i save it, for example, i've been trying to mod simdulgence's realgen penis for a while now and i thought i had to rig everything again but today i decided to test something and i simply exported the mesh to blender and saved it without changing anything then tried importing it back to the package, again without changing anything and then ingame the penis floats away from the body even though all i did was export and save using blender, what am i fucking up here? it's supposed to be literally the same file