Hello Bruno,

your surfaces are not well connected - to check it use this script as a shortcut:
script:var gd = moi.geometryDatabase; gd.deselectAll(); var breps = gd.getObjects().getBreps(); for ( var i = 0; i < breps.length; ++i ) breps.item(i).getNakedEdges().setProperty( 'selected', true );
Open the options and go to the short cut keys and press add. copy the script under command in the free field. Choose a shortcut in the first free field and close. Now press the shortcut and look what happens - the script will mark open edges. Showing isolines is only possible for a single surface/face. You can use Curves - isocurves for that.

Is the idea for this cube made by Ernő Rubik?

Have a nice day
Karsten