Ever since rhino 6 came out, when joining crvs on grasshopper and extruding them the surface created is one surface rather than a polysrf with 3 independent surfaces.
the same process on rhino creates seams between the different parts.
It might not seem like a big deal but it ends up creating weird geometries than cant be exploded later which is annoying.
Hi - as far as I can tell, that is the same behavior on Rhino 5.
To keep those as separate surfaces in Grasshopper, extrude the individual input curves and join the resulting surfaces.