Bug? the damage bonus from Inevitable Demise does not stack?

From what I have read, all of the damage bonus should stack, but in-game value shows otherwise; only the highest value trait will take effect (at least shown on top of the status bar).

For example, chest gives 2000 damage at 10 stacks, head gives 1500 at 10 stacks, then the status on the top will show 2000 damage.

Can someone provide any clarification for this? because this is huge for PVP