There is no train from MTN to STD. The tracks from STD->MT run alongside MTN but it cannot stop there, that I found at least. The platform is only built for the direction of HRL. That's the difference I see with Nuth at least, as someone who has destinations often north of the line maastricht-heerlen
You're right, a loooong time ago there were, but the removed the platforms on that line.
But, you're only a few minutes from Maastricht, so that shouldn't add too much time, and you get to get a good seat, since it's the start of the IC journey ;)