A type of ico in the RC file, and "ico" in FindResource call, should result in a custom resource type. "ico" is not an existing resource type (https://msdn.microsoft.com/en-us/library/ms648009(v=vs.85).aspx), which means it's being imported and compiled in as a regular blob in the resources. This is why it gets found when finding it, there's no special handling going on and you're matching the RC type when finding it.
ICON and RT_ICON will do it's own special handling, and I've found that icons can be pretty tricky when used as resources. Have you made sure that the icon is valid and supported (no odd sizes/laters/color depths/etc)? Open the icon in Visual Studio and make sure it loads/looks correct, and try actually saving it there as well. Make sure you save it explicitly (Save As and overwrite, or save to a new name), otherwise it might not think it needs to save.