Pete,
When I first read this, I envisioned this split along the pipe, rather than at the seam; Reading again, I see that's what you had said.
How does this muffler and pipe fit? What I'm asking is, does it seem to line up to all the mounts or do you have to torque it into place? Maybe there is stress on it when bolted in; then vibration causes it to crack at the weak point.?.
This is the only picture I have of my muffler, but you can see where the strap attaches; it's underneath and further back, connected to posts on the swingarm.