Here's an example activity:
https://connect.garmin.com/modern/activity/1562840623
EDIT: Nuts... I put the wrong link up there. The bug did happen with that run, but after the bug happened I manually typed the distance back into the activity. Here is another activity that I edited, just to see if I could recreate the bug: https://connect.garmin.com/modern/activity/1518152296
Note - It was the "full edit" page that I used to change the title, i.e. the pencil icon in the upper right of the activity. I typed in the new title and hit save, and when it brought me back to the activity the distance was 0.0.