This is the kind of thing that could very well happen during a pre-delivery inspection on a brand new car. Face it, the Mirage is the red-headed stepchild on any Mitsubishi dealer's lot, nobody wants to waste any time on them. "Hey, Joe, we got 5 new Mirages in yesterday, have you done a PDI before? No? Well, here's a checklist, go do them. If you have any questions, Bob will help you, don't bother me."
Or, it could actually be a supply chain problem, and the coolant they're using has the occasional bad batch?
Or, it could be something leftover from the manufacturing process of the heater core that leaves a residue that reacts with the coolant.
Sadly, it's one of those mysteries that we'll probably never learn the answer to.