A node_box with rounding=0.0001 or greater is drawn, but with rounding=0 it disappears. This is because it is drawn as the convex hull of four cylinders, each with radius=rounding, one at each corner of the box. If the cylinders have radius 0, they cease to exist. You could fix this by changing node_box.py from:
c=c+'translate([-'+xo+',-'+yo+',0]) cylinder(r='+rounding+',h='+zo+',$fn='+fn+');'
to
c=c+'translate([-'+xo+',-'+yo+',0]) cylinder(r=max(0.0001,'+rounding+'),h='+zo+',$fn='+fn+');'
and similarly for the other 7 cylinders.
This is in GraphSCAD 1.0.3 on Ubuntu 16.04
Subject: box disappears if rounding=0 
Wed 8 Feb - 15:48