Mine has started doing this too. It only happens with a walk, and not all the time. I will have a GPS tracked walk with time, but zero distance covered. Fortunately, I can go into Strava and correct the distance and then plug that into Garmin Connect (shouldn't need to do this, but it works). However, I'm worried there is something wrong with my device and would NOT be happy if this happened on a run and couldn't fix it manually.
