I got the steam key, but I had no place to enter it on Steam, so I had to buy it again. There was literally no way to use the key - no place to enter it, no option offered. To wit: I had made a whole separate steam account, which uses the same e-mail as my itch.io account, so I don't think that was the issue.
I know it's not a lot of money, not at all, and I was happy to buy it again, but it's just the principle on the thing.
Weird. No one else has had issues. And yeah, sounds more like an issue with Steam on your computer, than the game. Did you follow these steps: https://help.steampowered.com/en/faqs/view/2A12-9D79-C3D7-F870