How was it in the end?
Sorry the cook didn't go as planned, how was the end result? That's the important part. Way more important than this temp or that temp. The meat didn't care about the temp, it's already dead! Not to muddy the waters but some folks do high heat butts and brisket. We like temp consistency for predictability of finish time and sometimes as a method to yield a certain result, but if a butt's done early, great! Into a cooler topped with blankets/towels it will hold for hours on end and just keep getting softer.
Having said that I understand having a goal of some temp, say 225ºF to see what it's like for yourself and being disappointed because the cook didn't go as planned. Maybe next time.
That your WSM might have air leaks if it's that new could certainly be a culprit. I wonder if the burn was stabilized. Did you try to stabilize the temp and if so, like Todd asked, how long did you try to stabilize the temp for? Did you set the vents as you did when you were using water?
When I ran waterless my lower vents were 1/4 - 1/5 open by my eye and top was wide open. I'd often spend a couple hours monitoring before I was comfortable it was stable. Wind can play tricks on you too .. a mild breeze when you went to bed but not when you checked it.