I've noticed this just recently with Fenix watches.
I did the Moscow half marathon on Sunday and at one point my GPS track goes doolally (whether GPS, GLONASS OR GALILIEO). This has happened in previous races around the same area with different GPS settings.
I had a look through other peoples tracks and found that those with Forerunner watches had perfect or almost perfect tracks while the Fenix users all had the same glitch at the same location.
Look at the middle section of my track and you can see my problem: https://connect.garmin.com/modern/activity/3962194639
Forerunner tracks:
https://connect.garmin.com/modern/activity/3962066644
https://connect.garmin.com/modern/activity/3962128444
https://connect.garmin.com/modern/activity/3962141385
https://connect.garmin.com/modern/activity/3962146938
Fenix tracks:
https://connect.garmin.com/modern/activity/3962302484
https://connect.garmin.com/modern/activity/3962118058
Why why why?????