Garmin Instinct 2 GPS Pace and Distance Major Inaccuracy

Hi 

Just completed a 12km trail run and noticed major pace and gps problems. I run this same route once a week and my pace and distance are always fairly consistent and accurate. My watch always reads 12km at 5:20 - 5:50/km pace splits. 

Today my watch started going bonkers. Km 1-4 was measuring normal at between 5:15-5:45/km pace split. 

Starting at km 5, it started giving me paces between 3:08 - 4:30/km splits. It gave me a final distance of 14km. I have never run that fast even on downhill roads. I literally run the same trail start and stop same place no deviation once every week. 

Is it a settings issue, a software issue, a hardware issue or a bug?  

I am using theInstict 2 Solar, trail run profile, GPS + GLONASS and every second gps recording, 3D speed and Distance, software v9.27. I have not changed these settings ever.