Behemoth part's seem to be unbreakable if you're too far below their level. It seems to be about 10 or so more levels of difference between your weapon and the behemoth and it will just not be able to break parts, even if you spend the entire fight focusing one part, it will die before any of its parts break idk what causes this but it seems consistent with all weapon types vs a hellion leg that was about 12 levels higher.
They should scale with heath. Head and Tails need certain weapon types to break them now. So head requires a smashing type weapon like hammer, or Thundersoul sword. Tail requires slashing like Silver Sword, Chainblades etc. all other parts should break equally relative to core health.