Pretty sure this has nothing to do with RNG. Thrawn AI uses fracture on someone who probably had a buff from before, or who he determines is the highest priority target, assuming there's no higher priority to cleanse someone. So it's very likely that he'll always fracture the same toon, unless you ability block him, or increase his cooldowns and such. Unlike basic attacks, certain skills always target toons with highest health, or highest turn meter etc. It's part of what makes the AI competitive. If it was always 100% random, the AI would NEVER win.
Thoughts?