Well you'd think so. They do know about it.
But I suspect Garmin are losing interest in Mapsource - it's not great software and the obvious gaps in its capabilities haven't been addressed at all in the newer versions. They no longer ship Trip & Waypoint Manager (the cut-down version of Mapsource) with their newer GPS models even though I struggle to see how a new adopter would manage without it.
I got hold of a Dakota yesterday - no software at all in the packaging, not even a USB driver - my copy of Mapsource recognised it and worked with it straight away.
There's a workaround for the GPX problem.
If you start your Mapsource session by opening an existing GPX file - then you can go on and do whatever else you want, File New, etc etc, and then any Save As GPX will work OK.
Easy when you know about it.