Bug, Oversight, or Intended Behavior?

More than likely. Though if I was a Star Child in assassin territory I'd get a 100% Poison Resist item. Since nearly everything has coral armor or weapons and poison is annoyingly effective 1 on 1.

If you haven't, put a starshine cap on your Star Child (if you've invested alot of gems in his equipment already) and have him teleport back to labs to pick up specific items he needs for his assassin routines. (Anti-heat for Abysia, anti-poison for Atlantis, etc)