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Might improve if you turn on GPS+ Glonass or gallileo if you've got the option (and haven't already!). Using combo of two gnss systems tends to reduce accuracy problems a little.
I'm starting to suspect Garmin of planned obsolescence. My last two Garmin Fenix have seemed to get gradually less accurate over the course of years, even after a hardware reset. Don't ask me to prove it though! Just seems that running under trees or next to a tall wall is having more of a negative effect than it used to.
My garmin never seems to measure the same distance twice on the same route