I ran a 10,000m race on a 400m running track last night, and lo and behold my Forerunner 935 actually recorded 10.01km / 6.22 miles. My distance / pace accuracy on the track has previously always been poor, with the GPS usually overestimating the distance. I remember doing a 3,000m track race in 2016 and recording 3.26km. Has something changed? I'm very happy as it means I can now rely on the watch for pace info.
My settings are GPS/Glonass with 1 sec recording, with the latest 9.10 firmware. I run in the UK.