Just incase those have found this thread via the title and only read the last few comments. If you’re on a computer, then you should be able to receive the .fit file through the raw output files which it leaves on your machine, found in \documents\activities\zwift\activities.
If it says the file is corrupted, make sure you’re selecting the .fit file around the time you started the ride, instead of when you finished it. Then you’ll be able to add to Strava and Garmin as normal. Marie, to add a ride, go to the top right hand side of Strava dashboard page once logged in on a desktop/computer (not mobile), then select ‘upload activity’. Here you can upload your .fit file.
Sorry if you’re on an iPad/Apple TV, I hope they fix the bug and you get your rides back. When I first had the issue, I added the ride manually (if it’s not on Strava, it didn’t happen, right?), where you can include distance, speed, elevation, time, etc. The only thing you’ll be missing is HR and Power data (which is annoying I know).
I hope this helps at least one person.