itch.io

I don’t have Unity installed right now, but that probably means that they decided to re-set the icon on their own.

You would need to add a window message hook that intercepts the attempts to change icon and passes your saved icons instead.

In the GameMaker/C++ version that looks like the following:

LRESULT window_command_proc_hook(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp) {
	if (msg == WM_SETICON && window_icon_hook.enable) {
		HICON icon;
		switch (wp) {
			case ICON_SMALL:
				icon = window_icon_hook.icons[0];
				break;
			case ICON_BIG:
				icon = window_icon_hook.icons[1];
				break;
			default: icon = NULL;
		}
		if (icon != NULL) lp = (LPARAM)icon;
	}
	return CallWindowProc(window_icon_hook.base, hwnd, msg, wp, lp);
}
void window_icon_hook_ensure(HWND hwnd) {
	if (window_icon_hook.base == nullptr) {
		window_icon_hook.base = (WNDPROC)SetWindowLongPtr(hwnd, GWLP_WNDPROC, (LONG_PTR)window_command_proc_hook);
	}
}

As ever, all function signatures and typedefs can be taken from pinvoke.net