I decided to go farming in Helgrind Warren (for the Life minotaur pet) and this happened:
I cast a TC Franklinstove, which puts an elemental shield (fire/ice/storm) on the caster. Immediately following this, the first Fire boss (Runed Annihilator) cast a fire shark on me.
While everything looked normal (the spell displayed Fire damage and resist, as it should), the shark broke my ice shield, rather than the fire one. Later, the same boss cast a meteor strike on me, and it still did not trigger the fire shield.
Not game-breaking, obv., but it made me wonder: if boss had one of those -80% ice shields on him, would a fire hit from my spectral blast activate it? Did my Franklinstove and I break something, or is the boss AI just messed up?