The latest patch notes indicate that this sigil was bugged and always offering a damage increase when in fact it offered no increase at any time when I tested them 4 months ago. I tested them by dazing, stunning, knocking down, blowing back and petrifying and found damage did not increase whatsoever while the target was afflicted.
A few moments ago I tested this sigil again and found it still does not increase damage. I did this by going to the neutral Dolyaks just west of the northern camp in WvW borderlands and repeated the same attack while either applying stun or knockdown. While the target was either stunned or knocked down the damage dealt was NOT increased.
By comparison, I equipped both Sigils of Force and Sigils of Night (while it was night) and they both increased damage by the expected amount. Please look into this.