I had to toggle on "Indexed Attributes" and include the Normals attribute (N), on the SOP Import LOP. This same option would also need to be toggle on for the USD ROP.
With the normal attributed indexed, Cinema4D was able to generate the appropriate normal tag! What gets me is that Blender / Keyshot didn't require this to pick up on the normals. Still odd, but it's working now and I'm not questioning it.
