Log in to your Steam account to get help with your Steam games, contact Steam Support, request refunds, and more. Help is also available if you can't log in, need to reset your password, or recover a hijacked account.
After you click on the game, make sure you use "I have a question about this purchase" and not the "I would like a refund" button or it will automatically deny.
In the notes ask for a refund and include a screenshot of your save file sizes and a link to a news article about of the damaged save files and state you would like a refund as all your playtime was damaged by their bug. If enough people ask for a refund it will be repaired faster and if you wish you can always rebuy the game.
Don't be patient and hope they fix it anytime soon. Honestly the files are reportedly 'permanently' damaged so you may never get your progress back no matter what they do. Speak with your wallet, that is the only language the people at the top can speak.