AE 17.5.1 shadow catcher not invisible

If you want a shadow catcher to only show the shadow you should start with a 100% white layer for the shadow catcher. It will help you visualize the shadow. The layer must be 3D, the comp must be set to Classic 3D. For this comp, I set up a white solid and positioned it to fit the perspective of the shot, added a shape layer, made it 3D and positioned the shape layer in the middle of the sidewalk, then added a spotlight so I could diffuse the shadow. The first screenshot shows the setup with all modified properties of the layers. If you turn off the 2D background the shadow catcher will appear to be a white solid. When the 2D background layer is turned on the white disappears because when Cast Shadows only is turned on the layer's blend mode is set to multiply even though it does not show in the Modes column. 

So to review, the comp must be Classic 3D, the light must be set to cast shadows, the shadow catcher must be set to Accepts Shadows Only, and the background layer must be 2D. If you need the background layer to be 3D there is no reason to set up a shadow catcher.