Hi Pilou - I seem to remember that ZBrush does some kind of automatic unwelding when it imports .obj files in some circumstances - I think it is when the welded points have different vertex normals for the 2 faces that share the point.

Since ZBrush is doing this unwelding on import, you'll need to do the weld operation inside of ZBrush to fix it up - I don't know what is called, look for something like "weld", or "merge vertices", or "glue vertices"...

- Michael