Hi,
I recently completed my first race on Zwift and was experiencing internet issues throughout the ride. These issues were at my end. The upshot is that none of the ride made into Zwift but the activity .fit file was available on the laptop and was not corrupt. So I was able to upload the ride successfully to Strava and Garmin.
Can you consider adding a feature to upload rides from Zwift activities folder where they have failed to upload previously.