I just ran a quick a test with the c_sharkhm model, imported it, did nothing to the model, exported it, imported the new model. Nothing changed.
Ran another test, imported the model, moved a few verts around, on a mesh that is not skinned, exported it, imported the new model. Again no changes.
[EDIT] Maybe I have found the issue. Third test, I did a 'Reset X-form' before exporting, imported that new model and all skin weights were now screwed up.
This looks as if it may be a 'Reset X-form' issue, and something that has been around for quite some time.
Dirty fix would be to unselect any skinmeshed items prior to performing a 'Reset X-form'. [EDIT]
Not sure where to find the giant slug model. If you could point me to that model or provide me the one you used, I'll take a look at that.
Was going to release, but I'll wait a little longer to see if I can track down this issue.