Instinct distance different during and after activity

Hi all - hoping for some help / advice on this!

I recently got a Garmin Instinct and have been having an issue with the distance calculations during an activity.

Whenever I use the watch to track an activity, while the recording is taking place, my Instinct drastically undermeasures the distance covered. The under-calculation generally works out to about 40% and happens across the Run, Hike and Walk activities (haven't tried anything else). Because the distance is under-calculated, this then means my pace is completely off, meaning all I can use to gauge performance is the total time elapsed.

I bring my phone with me on these activities and once I complete the activity and view on my phone, the distance and pace are calculated correctly within the Garmin Connect app on my phone. So the correct information is being tracked/calculated somehow. 

Has anyone encountered a similar issue with the Instinct? Any help or info would be really appreciated as I really like the watch other than this issue!

For reference: 

- I am an active enough runner and have been tracking activities on different smart-watches/apps for about 3 years now so it's not a case of me simply misjudging distances

- I wait until I get the all-clear from the GPS icon before starting an activity

- My activities are not in big urban areas with lots of buildings

- I have tried with/without 3D speed & distance enabled

- I have tried: Normal (GPS only), GPS + Glonass, GPS + Galileo settings