Hi. Not a builder (looking to do some builds for some class projects) but I am a welding instructor at a local community college. Also a CWI. Looks like a heat issue. It looks from the pics that the heat wasn't even. The area of the failure shows a not hot enough adhesion. And there should be a bit of capillary action of the filler into the mitered joint and I don't see that. So uneven heating and not enough heat at the failure point. My opinion.