Obviously, it is extremely common for people to occasionally forget to turn off their watch immediately upon completing a workout.
Does Garmin not have an engineering team working on the web site? This has been a known issue for years, and it's strange that it wouldn't have been corrected by now.
If you use Strava, you can crop the activity there, export it as a TCX file, then manually upload it into Garmin Connect as a TCX file. Then you just need to delete the original (un-cropped) activity in GC.
I am assuming "no" given the answers above, but can an activity be split? I did not reset my device and so it is treating 2 rides as 1. Can the "1" activity be split into 2?