The real culprit is geometry, and the needless insistence on placing the lugs such that they're all scrawny right there (instead of all being equal to each other, if not 'pleasingly' symmetrical. Or just shrink the extractor a hair & spare the bolt
It's kind of funny that in all the years, and all the iterations of strategic reinforcement the AR has undergone, the bolt is unchanged as far as I know.
Another thought I had is that when Alexander Arms had their bolts modified to make them proprietary (I can see literally no reason for changing the headspace spec like they did) someone didn't use a properly radiused cutter or left a rough surface finish, or an un-peened or un-case-hardened finish (knowingly or not), and ended up with that portion of the bolt being a little 'inconsistent' with the rest of the part, contributing to a stress riser. There're reasons for doing things like your finish shot-peening or or case-hardening or coatings all at the same time, namely that it's really hard to be as consistent a second time.
TCB