It does look more like external corrosion to me too. But the other metals close to the tee and the one crimp band has no corrosion at all, so I'm thinking it is a quality issue with the affected parts.
This pipe does sit above the gas furnace. And the gas hot water heater is also close by. I'm mostly worried about having to repipe the whole condo if it's bad components that were used.
In that case go inspect one of these components which is far away in another location. Might be in a wall though. A good place to take a peak would be in the back of the cabinet under a sink in a bathroom. Those usually don't have backs - the wall is directly accessible from inside the cabinet. If you have to poke a little hole in the drywall to look, the quality of the repair in a hidden location like that doesn't need to be stellar, for instance, you don't really need to paint over the patch. If you are really lucky there might even be an access panel in one of those locations.
