I have strange distance behaviour on my Forerunner 35. Sometimes I'll start recording, and it will just not record distance at all, however when I view the activity in Connect, the GPS trace and speed is accurate, but distance 0.0km. Sometimes the distance is correct once synced to strava, sometimes not.
Sometimes it'll record SOME distance, but not as much as it should, for example this is about 5km and it's gotten 2km.
Sometimes it works perfectly. Sometimes turning it off and on again and restarting recording fixes it, sometimes not. Seems to be no reason to it at all. I've tried restoring to factory defaults.