After looking at the capx, I know what the issue is. You are setting the slider.y to touch.y. But they are on different parallaxes. The slider is on 0 parallax, but the touch works on the entire layout which is using 100 parallax. That is why when your sub goes down, your slider wants to go down too, because touch.y is actually much lower than the limit you imposed on it, but because of the limit, it won't go lower than the slidermarker.
Fortunately, the solution is easy enough. All you have to do is instead of "set Y to Touch.Y", set it to "set Y to Touch.Y ("UI")" This will tell C2 that you want the touch value that is based on the UI layout.