(Known Issue) GPS accuracy on Jan 01 2021?

I said in my opinion and YMMV, but if you feel it to be a bit harsh, can't really help there.

Probably it also has to do with how long has the activity been, mine was around 3 hours and a lot happens during that how satellites move and so on.

DC Rainmaker's take

https://www.dcrainmaker.com/2021/01/gps-accuracy-impacting-devices.html/amp

The distance has moved from 400m to 12km now, Just tested some Minutes ago, total wrong start point total wrong interpretation of movement (4.5 km instead of 1.6km)

Fix !!!

For me tested with long GPS fixing

Affecting Garmin, Polar & Suunto Watches (Sony GPS)

https://mightygadget.co.uk/garmin-gps-accuracy-issue-caused-by-expired-cpe-january-2021/

[Update 3rd of January 2021] Fix rolled out for Garmin
It now looks like Garmin have pushed the CPE file update to their users, if you have phone syncing on, this should update automatically. You can also manually update via Garmin Connect
If you check System > About and CPE and it shows expired, the update has not occurred. If you can't get the update for whatever reason, you can still leave the watch for 5 minutes or so tracking an activity to try and let it update it self.
The update will have been provided by Sony, so other brands affected by this problem will likely be updating currently, or soon.

[Update] Official Garmin response
Please be aware that we are aware of the serious GPS issue that 2021 has brought upon the Fenix 6 series watches and we are investigating reports. The issue seems to be impacting not only Garmin but some of our competitors too.
On your watch, if you go to System > About and CPE shows Expired, please give 5 full minutes outdoors with no movement to see if your CPE file will update for you to CURRENT.
Even if this doesn't fix the CPE you should allow the watch to GPS soak to receive a full satellite fix. This means to wait for an additional 2-5 minutes after the device reads GPS ready.

My CPE status is CURRENT I don't know if this happened while I was running, but I did 8 Km .. I waited for the GPS to be ready before I started running still the gap is almost 1 Km at the start and most of the route recorded was 100 to 500m away from the real one.. I'll try tomorrow again

This should be fixed now according to 945 FAQ pages.

After January 1st 2021 My Garmin Watch GPS Track Accuracy is Incorrect

Garmin was aware of an issue that may cause your Garmin watch to log GPS tracks on the incorrect streets for a large duration of your outdoor activity.  This issue has been resolved. Your watch will automatically download the latest satellite data to your watch over the next 24 hours.  

To immediately update the satellite data on your watch, perform one of the following:

• Sync your watch via Wi-Fi
• Sync your watch via Garmin Express
• Power your watch off and then on, wait a minimum of two hours, and then manually sync via the Garmin Connect app

(I hope this is not a duplicate post.)

Fixed for me now (Jan 3, 2 PM). Very cloudy in Chicago, so it took a while to lock in, but once it acquired the GPS signal, it was solid. Yay!

My husband is having a similar issue. Yesterday we did a 1x1x24 (1 mile on the hour, every hour for 24 hours).  His out & backs were way off; while mine on the 935 were fine.  He's out now and his live track is showing him way away from where he actually is and I'm pretty sure he didn't swim across the river twice during his 5 K run!

Have you read the thread to understand what happened, why it affects the 945 and not your 935, and how to correct the issue?