What you are describing is exactly what BKVP said. Every set up is different. What works good in your house may not in another.Taking all of the different setups and modifications into account I still can't see how this can be purely a draft issue. Perhaps if the draft is ramped up enough it will eventually overcome whatever defect is allowing this to happen, but I just don't buy that this is 'normal' in any way.
I have a 15' single wall in exterior masonry chimney, in a mild climate, and both stoves I've run up this chimney have had zero issues with smoke leakage. The non-cat insert I had before was vented straight up, and I'd almost have to whip the door open just to get smoke spillage, and the cat stove I'm running now runs 2' horizontal out the back, into a 90 then up the 15' from there. It's 50 outside right now and I'm running low cat burn at 370F with zero smoke inside or outside of the house, the way it should be. My setup now also has seams between the back of the stove and the 15' vertical run, none of which are sealed beyond how the pipe fits together.