Can't explode extrusion faces


I don’t understand an issue with an extrusion :

I created two arcs (with two differents radius but the same center), arcs I connect with two lines.
I extrude it (with rhinoscriptsyntax ExtrudeStraight) and close the volum with CapPlanarHoles

Then, I’d like to chamfer some of the edges. But whatever I try (explode polysurfaces, duplicate faces from brep), well the surfaces of the initial extrusion makes 1 surface or 1 face, and the two caps 2 more.

Any explication on why I can’t separate the 4 surfaces from the 4 curves ?

I found my way by extruding all the curves one by one then joining the surfaces, but I’d like to understand what’s going on