Apparently Mono only accepts icon.bmp, not the current icon.ico.

Solution reported by Nanogamer7 on Discord.

Fixed via #910 for the upcoming SMAPI 3.18.5. Thanks to @Datrio for the fix!

See proposed fix in reddit discussion.

The fixes I've tried haven't worked, and I have no plans to investigate further, so I'm shelving this. I'd be open to considering a pull request to fix it, though.

OK, on a Mac, it definitely is being changed during launch. It starts with the correct icon, then changes to that. What I don’t understand is where that icon is even coming from.

I’ve figured out the problem: http://community.monogame.net/t/app-icon/8283/3

Can you try setting the icon as an Embedded Resource?

Well that's progress, thanks! I added an embedded Icon.bmp, and the taskbar icon disappeared:

That’s definitely better than an incorrect icon, at least. Is it possible to build and test that without having to install the 1.3 beta? If so, I can do it now to see if the macOS dock icon is fixed.

Yep, you can checkout the stable branch (which is for Stardew Valley 1.2) and cherry-pick the commit from the wip-linuxmac-icon branch.

Turns out the compiling process is way less intuitive than I expected. I can’t figure out what to give as an argument to mono. I really don’t want to install Visual Studio for this, so I’m giving up on that.

I poked at this a bit and couldn't get it to work on Linux, regardless of including an icon.bmp or Icon.bmp, setting it as the application icon, or including it in the build output. The launcher icon works (it worked fine with icon.ico too), but the taskbar icon is unchanged:

Several attempts to change the default icon have failed for various reasons, and I have no current plans to address this, so I'll close it as won't fix. Pull requests are still welcome if someone wants to contribute a fix, but otherwise I think we can just accept the default icon.