When I run on the treadmill and calibrate the run distance before saving the run, the mileage that gets uploaded to my Strava account ends up being the pre-calibrated mileage, while the mileage that is saved to my Garmin Connect account activity is the correct, post-calibrated mileage. There's no way for me to update the mileage on Strava, so I have to delete the entry and manually add a new one, which then doesn't have my heart rate info included in the run.
I'd hope this is something the dev team could address with a fix at some point.