Yes, that’s what confuses me. To make this trick work, it needs special circumstances. Since I build every new surface with hidden isocurves to make the model appearance as clean as possible, there is no quick way to see the isocurves quickly by simply selecting a surface, polysurface or a sub-object surface. Anyway, I guess I will have to wait until Rhino 7 or Rhino 8 for this option to be added. Maybe.
One way to somehow make it work is to turn on the isocurves by default in the options menu, then modify every existing default and custom display mode to have the isocurves hidden (Rhino options > View > Display modes > “name” > Objects > Surfaces > Surface isocurve settings > Isocurve width = 0), and create another custom one which will show the isocurves and assign a key or keyboard macro to it.