If you look at the op's pics in the first post, it doesn't look like the insert has been pulled out. It only looks like the surround over the stove was removed. Furthermore, I would say it has been like that from day 1. Chances are, there was enough temperature differential (inside/outside) when cold to make th draft pull even when not in use. When it was first lit, it was drawing most of the smoke, but some of it still leaked into the room. As the temp built up in the stove and was released, it increased the velocity of the draft, therefore sucking it out more efficiently.