I'd say, bug of a different sort.
Another thing I currently got in SP - not sure whether it was in a short list: the Forge Lord consistently refusing to cast on himself Immolation (the spell that gives fire aura but burns non-immune caster as well) even though he had FR 100 or 150 in some cases. He prefered to cast Ironskin above existing Stoneskin (!) (there were lightning-casting enemies on the field) or, if Ironskin was scripted, cast Bladewind or Falling Fires (iirc) instead.