garmin instinct solar distance incorrect

i have a strange problem with my instinct solar

just done a known route that is about 8 miles and watch as only logged 4 miles , it seems everything is running at half 

distance

avrage speed

max speed

i can correct in strava but its rather annoying

tried so far

factory reset , was ok for a bit but prob has now returned

all gps settings

3d speed and /or distance