When you use commands that divide a volume, Rhino creates different objects for them.
But if you select a sub face of an brep object and delete it with the Del key, Rhino create just one open object.
Where the command ExtractSrf create two.
Hello - yeah… a disjoint Brep is a little peculiar - I’m not sure what should happen here, but I think I’d expect two breps. Meshes are different though - disjoint meshes are ‘ok’ and expected in Rhino.