I found solution about this into the
Autodesk 3ds Max Services & Support. Interesting is that this problem applies to all 3ds Max versions (!).
Issue: When you export to 3ds, the geometry is exported but some vertices are not welded together.
Solution: To export welded vertices to 3ds:
- Select the geometry
- Right-click it and select Convert To Editable Poly
- Press “1” to go under the vertex selection mode
- Press Ctrl-A to select all vertices
- In the Parameters > Edit Vertices rollout, click Weld to weld all vertices. Leave the weld value to its default.