GPS accuracy is out after being correct for years...

Mine completely out all of a sudden as well.

did you find a solution?

I am experiencing the same issue here (netherlands), especially the first kilometer or so. Even when waiting for the FR645 to signal me that GPS is good.

I was using GPS + glonas, but now switching to GPS only or GPS + Galileo; see if this helps.

Same issues here also in the Netherlands. Last run without issues was 14 August, the issue appeared on 20 August (+21 and 23). Normal GPS seems to be the issue, especially after running beneath a bridge or something the track is way way off and stays that way. Always had GPS + GNSS (or Galileo) but since GNSS/Galileo is working fine the track/run is completely *** up since the watch thinks I'm running between the GNSS/Galileo correct position and the GPS flawed track.

This is definitely a software/GPS issue and not broken hardware since all these reports popping up at the same time is not a coincidence. For instance on reddit: https://www.reddit.com/r/Garmin/comments/wtv2te/garmin_645_gps_issues/ and a Forerunner 255 user on dcrainmaker (last comment on the bottom of the page): https://www.dcrainmaker.com/2022/07/gps-accuracy-issue-impacted-garmin-suunto-and-polar-watches-over-past-week.html

Clearly something wrong.

My first occurrence was 20 August as well, followed by poor GPS on 21st and 22nd (I haven't run today as I'm fed up).

How do we get Garmin involved in this discussion?

Looked into some more details of past activities. I do see issues of GPS being less accurate on 18th onward, and serious misses on 21st. Especially when running underneath a bridge or so. Hope Garmin can deliver a quick fix....

I filed a support ticket at Garmin, would suggest everybody does this, to make Garmin aware.

Now have a few tracks logged with GPS only; large misses are absent; but tracks are very inaccurate (>10 m)

mine is out by 30% HR is also out as is the altitude-  i noticed this on my 10 mile run this week which i do on a regular basis- it had me at 7.81 miles so way out and very frustrating- going to mess around with the GPS....

I was contacted by Garmin to do the following: with garmin express hvae the phone updated to latest SW versions and perform soft reset (on/off for 5 - 7 seconds; (watch is off), on/off for 25 seconds; on/off for 5 seconds: phone is on and soft resetted).
Then 30 minute activity to calibrate.

results:

1) with GPS and Glonas: jumps in position not observed, but position is very inaccurate

2) with GPS only: no jumps, but inaccurate position, especially next to high buildings, under bridges

3) with GPS and Galileo: no jumps, position reasonably accurate, except in the beginning of the track (> 10 m off), but end ok.

Thanks for this, will try this now

Switched back to GPS + Galileo, also switched 'smart data recording' to 'every second' and problems seems to be gone. It's either this or the watch has a new (good) EPO/CPE update. Need to do a few more runs to be sure it's working 100% again.