I've got the same problem. Flew to Duna last night, set up an orbiting probe at 125Km, could hear the probe making it's beeps as it passed the various cells but the grid was not overlayed on the planet. When I moved the camera around so it was inside the planet I could get the grid to show occasionally and see the white and green cells highlighted. The probe is doing a scan correctly but the overlay seems to be broken, on Duna at least.