Expand appearance typically works this way with stroked objects. The stroke ends up as an extra compound path inlining and outlining the original shape.
Alternative: use the Path Offset effect. The effect will give you three options for corner effects. When you expand the effect it will consume the original shape. No groups to break apart or extra paths to eliminate. For clean outlining effects I'll copy the original shape to the clipboard or place a copy of it on another layer. Apply the path offset effect and it's done. Nothing more to edit.