Elemental Shields - Possible Bug

When an enemy has an elemental shield equipped, they still take 90% less damage, even if you cast a spell from a different element.
Is this intended?

Isn’t it against your creature class and not spell class?

Not sure, that’s why I’m asking. Part of the reason I use different schools of spells on creatures is to overcome weaknesses.
Otherwise, the Chaos Mage perks don’t make a big difference at all.

I think shields work vs class, and the chaos mages perks allow you to use a wider range of spells basically, it’s a really useful perk