If your application uses raw NT syscall (which Microsoft explicitly says is an unstable interface) and an upgrade breaks it, then the application certainly is at fault. Like, this isn't some incidental case where you accidentally depended on unspecified behavior or an official interface broke; if there's a big warning on an interface that says this thing is unstable and will break your code, and it breaks your code, that's definitely on you.

If there was no stable interface to Windows, that would also be Microsoft's fault.