I loaded your save too look at it. Looks like spells are free because the reductions stack additively.
-50% mantle of oceans
-25% path of the mage
-20% mage robes
This is -95% reduction in mana costs. So blizzard goes from 30 mana to 1.5 mana, which it rounds down to 1. Mass curse goes to .75 which rounds down to nothing. You can actually get this entirely to zero with the trait Affinity (-25%), path of mage (-25%), and Mantle of Oceans. Completely eliminates the need for robes or items at all.
So I'd say not a bug, only a design decision. Perhaps some of the reductions should stack multiplicatively? Maybe all item benefits should add, and be multiplied by the sum of spell benefits, and multiplied by the sum of all trait benefits?
In that case, the affinity + mage + oceans becomes (1-(0.25+0.25))*(1-0.50) = 25% cost spells
and your case of mage + robes + oceans becomes (1-0.25)*(1-0.20)*(1-0.50) = 30% cost spells
Both are still pretty sweet, and 'free' should be unobtainable with the traits, items, and spells currently in the game. Plus would open up the idea of traits or items or spells that reduce the use of specific spells, possibly allowing a full 100% reduction to be reached.
