At all ? Even in combat ? I remember long time ago that ZOS attempted to nerf it (lol )so it would only work in combat, but they thankfully they abandoned this idea.
It's bugged, it was mentioned on Nefas stream that it's happening to some people and not others. It's happening to Sorc ult reduction passive as well, they fixed that one by just removing the skill points and re-assigning them. Not sure if they tried it with the nightblade.
I only DD in wizard elf game cuz I like seeing big number
Tick Tock Tormentor | Saintly Savior | Gryphon Heart | Godslayer | Kyne's Wrath | Planesbreaker | Swashbuckler Supreme | Mindmender | Unstoppable