Sony-made GPS chipsets failed all over the world this weekend when a GPS cheat-sheet of sorts expired:
In general, the pattern of your route is correct, but it may be displaced to one side or the other. However, in many cases by the completion of the workout, it sorts itself out. In other words, it’s mostly a one-time issue.
The issue has to do with the ephemeris data file, also called the EPO file (Extended Prediction Orbit) or Connected Predictive Ephemeris (CPE). Or simply the satellite pre-cache file. That’s the file that’s delivered to your device on a frequent basis (usually every few days). This file is what makes your watch near-instantly find GPS satellites when you go outside. It’s basically a cheat-sheet of where the satellites are for the next few days, or up to a week or so.
I experienced this failure as well. I recorded two walks on my Garmin Venu, one Friday and one yesterday. In both cases, the recorded GPS tracks appeared about 400 m to the west of where I actually walked.
Because the issue started between 22:30 UTC on December 31st and 15:00 UTC on January 1st, I (and others) suspect this may have been bad date handling. Last year not only had 366 days, but also 53 weeks, depending on how the engineers configured the calendar. So what probably happened is that an automatic CPE update failed or appeared to expire because the calendar handling was off.
Dates are hard.