I believe that is simply brittle paint cracking under normal BB shell stress. I'm sure someone will come up with a pic that proves me wrong, but every time I've seen a joint fail it's been the tube that cracks/breaks, not the lug or BB shell. It makes sense, as the tubing is much thinner than the shell or lug.
I've never, ever seen or heard of a shell cracking/failing like this. There's always a first time, of course. Take a careful test ride if you can, see if you can detect anything.
If that's original paint, I'd hate to take it off in the area of the crack just to disprove what is extremely unlikely. If it's truly cracked, a short spin around a parking lot should give you the answer.
DD