Bullet stability is a subject I've spent quite a few hours researching, and there are a lot of misconceptions as to just what it is and whether or not there's such animal as "too much of a good thing" when it comes to stability. I had never considered the possibility that the core could be liquified and it makes perfect sense that it would compromise bullet integrity.

You nailed it on velocity being as much a part of the stability question as rate-of-twist is---it's about RPM.