I noticed this behavior recently when modeling some plugs to fill some bolt holes. There isn’t much to explain that isn’t shown in the video. The diameters for the plug shafts and the plug heads were sketched on the main plane and the shaft was extruded to the needed shaft length. A new sketch was started on the face of the plug “tips” and a new circle was drawn concentric to the extruded face and then shaped with fillets. Then I returned to the head end with the intent of extruding the shaft and head circles in a union extrusion and the video shows the result.