I just specced into the Riposte skill from the Melee tree. From the description, it is supposed to stack up to 50% more damage for the next skill, per stack of 5% damage per deflect. So it should increment up to 10 stacks from each deflects.
In actual gameplay, I can see the icon appearing on screen after a deflect. But no stacks are shown :
Does it actually stack ?
If only visual, then it's a minor bug, but it would help to know how many stacks there are before performing a skill or another.