They get added to the node network, but they are way off somewhere else on the node network view, and dont attach themselves to compute terrain, which is how tg2 finds out what to consider terrain.
You can usually only find them in node network by browsing for their name in the box of all shaders listed, and then try to move/reattach them.
I've noticed this before, and since then I've sorta just subconcisously always added terrain and then deleted it. Forgot about it to report on here.