Definitely sounds like something is being done to them ----pretty sure it's not a shaft integrity issue. I've shot Goldtips and CX Heritages and have found the CX's to be the toughest arrows I've ever shot-- wood, aluminum, or other carbons.
I tried using the 350's using 250 gr. up front, leaving them 30" long out of a 62# recurve--still too stiff! I now have the same set-up except using the 250's--and all is good! I've bounced the CX's off of all kinds of stuff because of crazy shots and arrows that don't know how to hit a target
Anyway, the only heritage I've had break was one that I heated (stupidly, accidently) to try and remove an insert. I've even bounced them off a rock or 2 shooting at half-buried groundhog targets back-stopped by a rock----checked em and kept on shooting them.
If you are shooting a bag target, any chance it's wore out and your groups are close? Maybe they are getting hit by each other while inside the bag? Not really any other idea about what could be causing this---jsut like to ramble on sometimes!