I think you may be dead on. A miss identification of HAZ seems possible. Its hard to imagine true HAZ problems with SS's unless a base metal with high S (or something) is liquating at high temp. Though I've never seen this. I could be wrong. Still, fusion boundary cracking leading into the HAZ with austenitics is not something I've seen either. Again though it could be maybe S diluting at the fusion boundary,as you say. Most hot cracking accomodates itself by pushing cooling stresses and 'junk' along the solidification front until the point of last solidification. I think we need more info. Uh, so that we can generate more speculation. Its fun. Cold cracking jumps the fusion boundary, but thats not something you normally see with austenitics.