Aria Stones and boss jewels

I am fortunate to have more then one boss jewel and recently put augments on all three but noticed an issue or at least I think an issue and that is the element resists do not stack. So I have two jewels that have dark resist +10 but only one of them actually works. I wonder if this is a bug or if it was part of some poor design? It makes no sense to have two jewels with lets say +10 dark if they both do not work.

Anyone else notice this and can maybe a GM respond to wether this is a bug that is being addressed or it is purposeful? It costs way to much for it not to stack.