There's this weird option to use a sneak attack as a type of attack, actively, by clicking on it. If you play a ranged character, pressing this button doesn't allow for other modals, magical arrows, spells, etc. to stack with it.
Initially, I was confused and disappointed as heck; my whole idea of an Ambush Ranger had withered.
Thankfully, it's all really just a confusion. Whilst just shooting from the Dredge Ambusher's modal, the sneak damage has been added automatically, as seen from the log.
Man, why is there such a button at all :D ?
Anyway, I hope I have cleared this out for others who might be interested in similar builds.