I get to add to this thread! So exciting.
I’m pretty sure this fail is due to my poor brazing skills at the time and the fillet not being a good fillet. You can see in the image how abrupt it ends and that probably caused this. I noticed the crack starting while on a trip and really should have stopped riding, but I was in Moab and couldn’t help myself to a few more miles of fun. oops.
Now the question becomes, “Am I capable of replacing the downtube and not messing it up this time?”