Slight update: The afternoon rid got logged for 2.99 km (out of 10). The 1030 seems to think there is GPS reception, as it's not warning for "satellite lost", but that I am not moving. When I toggle between GPS and GPS+GLONASS, or when I turn GPS off, I get the "satellite lost" message.
Have you tried resetting the 1030 to see if it fixes the problem? To avoid losing your settings and customisations on doing the reset, you can follow the procedure at https://support.garmin.com/en-AU/?fa...aC3xrXxmQj7U9A.
Sounds like you might be getting occasional speed readings, enough to confuse the 1030 into using the speed sensor over GPS, could your cadence magnet be tripping the speed sensor? Can you try moving the magnet a bit further away from the speed sensor?