Yeah, that's really odd. Both are definitely B6 metal, and you can, in fact, book both of those flights (1225/1027) on points separately--it's just that the website won't price them out as a connecting itinerary. More crappy B6 IT.

It's probably one of those situations that requires calling unless you want to just book separate segments on points and then have the check-in agent at MVY check your bags all the way through to BWI. You will probably pay a bit more in points (12,900 total by our calculation) doing it this way though, except you'll also save hours of your life not waiting on hold for B6.