Epix Pro Gen 2 shutting down / dies during an activity

I was wondering if someone within the community could help me with this.  On two separate occasions my Garmin Epix Pro (Gen2) Standard Edition has shut down / died during an activity.  This happened most recently while out on a 15mile hike.  At the start of the activity the watch had 15days worth of charge.  I was tracking the walk and using an uploaded GPX file to map the route.  Everything was working fine until towards the end of the walk (6hrs in) and the watch just shut down and died.  I was unable to restart the watch so it would appear the battery had completely drained.  I have checked all the settings and was using GPS / Satellites to "Auto Select".  The watch wasn't being used for anything else other than tracking the walk.  I have rebooted the watch made sure everything is updated. 

I have bought this watch to track training for ultra marathons (<50miles) and expected the battery life to be more than sufficient.  My husbands watch (Epix Pro (Gen2) Sapphire Edition was absolutely fine.  

Is there an known issue or setting which I should change or does this sound like an issue with the watch?   If it's the latter how do I contact Garmin Customer Services & Support?

Thank you in advance for your assistance.