I have this exact issue, and I have done ALL of the following:
Thoroughly verified all of my Steam settings
Uninstalled the game
Purged the game data from my Steam Cloud
Purged files and folders related to Stardew from every nook and cranny of my computer
Purged everything Stardew-related from my computer registry
Restarted my computer after wiping all of the above data
Reinstalled the game after said restart
And after all that, the game still deadnames me. I love this game and have cherished it with my friends for over 2 years now, but now I can never play it again unless this is fixed.
Hey! I just realized that I had this same problem (trans and my deadname popped up)
The only way I've managed to solve it is to make a whole new steam account for playing multiplayer with my friends.
If you log in to Steam on the same computer with your new account you'll be able to see your old account's games. To get access, you can request to borrow the game. You'll get a Steam Customer Service email to whatever email your old account is registered under. All you have to do is approve the request and you'll be able to play all the games in your old library on your new account.
Or you can just make a new account and get the game again but that feels like a waste of money ngl
If you're an achievement and playtime junkie like me, you can use the new account only for co-op games and log back onto the old account for everything else.
(Also my software engineer father is currently very upset about this and is literally combing through every stardew and steam file he can find so I'll let you know if we figure out how to actually change it)