If it's the same brown stuff that's on my tires (BFG Long Trail), it is definitely not the dressing. When I first bought them I noticed that just one of them looked like that, and the tire dealer told me that it should come off the first time I washed it. Since then, the other three started doing the same thing, and no matter what I use to try and remove it, it is still there. Strange thing is, my mother's 4th gen has the exact same tires purchased at around the same time and they don't have this issue.
I've been using the same dressing for a long time now (Chemical Guys matte) and have never had the issue before.