I'm using a Fenix 6X Sapphire and Garmin Connect android app v4.50
I've done a treadmill run today. I've saved & calibrated the activity which was a 9.09km run. For this run ice gotten a "Fastest 10k" record even though I didn't run 10k. Before calibrating the distance on the run it was over 10k, so my best guess is that the record was created based on the uncalibrated activity.
I've gotten these three for the same activity: 