Hi mogn,
This is because TG2 uses the "/" character to delimit paths through the node network. It allows you to create links to nodes deeper in the node network than the node you are editing. The reason the node preview flips back to the showing the whole terrain is that when you add the "/" it is looking for a node called N which is ( I think ) inside the PI node, and because it can't find that node the preview goes to its default display, which is showing the scene.
This is and isn't a bug. I think that perhaps shader name fields should not allow the entry of characters like "/" which aren't valid in names. I also think you shouldn't be able to enter non-numeric characters in numeric fields etc. as well, but I think Matt has a different view on this and is happy to let anything go. Essentially, don't use the "/" character in node names.
Regards,
Jo