I completed this workout today
https://connect.garmin.com/modern/activity/16944786210
Which was a three segment run of 2 miles, 12 miles, and 2 miles. When I originally created it in Garmin Connect on the web I accidentally made the last segment only 1 mile, so I edited it to 2 miles after it had originally saved and synced to my Forerunner 225. Then it resynced afterwards before I ran the workout.
When running the workout, after completing the second segment of 12 miles, the watched beeped and showed me with only 1 mile remaining for the last segment (it should have been 2 miles). The watch was also reporting I had already run a total of 15 miles (when I had only run 14). I ran the last segment and then an extra mile to get myself a total of 16 miles (which is what the workout was supposed to be) but as I stopped the watch reported I had run a total of 17 when I had only run 16.
Looking at the intervals view, Garmin Connect recorded the 12th interval of that 12 mile set with a time and cumulative time of 1193:02:40, which of course makes no sense. Garmin Connect also seems to have grouped the actual last 12th interval of the 12 mile segment with the remaining 2 mile segment (which you can see as the pace is different). On the map view, you can see it recorded the 15th mile as completed almost immediately after the 14th mile.
When I ended the run the watch reported a bunch of supposed new records, because it believed I had run a mile in less than a second.
I believe the cause is some kind of sync problem created by the edit to the workout, because that's where the problem showed up, but that's just a guess.