Probably because if your like me, your using somebody elses Flatpak install. It might have escaped you, but Itch isn't responsible for that. Hence the lack of a response.
Now, they do have an official Linux installer. (Ask me how I know?), and that too is just as broken (i.e. Cloudflare looping),.
There is a way to fix that apparently, if you can possess minor reading comprehension. The good news is it will fix the problems. The bad news, is it seems to break something inside SteamOS. The App will work as expect, with no CF Errors inside the Desktop, just not inside SteamOS. The App just never starts, it will eventually time out, after 2 minutes, and crash back to the launch screen.
My fix was to install my the Windows version under Proton, apply the same fix, and this works fine under SteamOS, and the Desktop.
Not ideal, but it works. But, it's in line with a my experience with the SteamDeck, for some unexplained reason it seems to hate native Linux Apps, and refuses to run them. Beit Games, or Apps.