HI Tom, that's a history updating kicking in.

You can avoid that from happening by disabling history updates on the extruded surface. To do that, before you hide the surface, select the surface and then run Edit > History and push the "Disable update" button.

Then the connection between the original curve and the extrusion will be broken and you won't see the behavior you don't want.


Another shortcut way would be to hold down the Ctrl key while you drag a point on the curve. When you hold down the Ctrl key and drag it will make a duplicate of the thing you're dragging on - that works when dragging on full objects or control points as well. The new duplicate will not be connected to the extrusion and you can delete the original curve and continue working on the new one.

- Michael