That is because you need to use the color coding system because ahg doesn't work with negative Gs. Go look back at the tutorial on the first page because I don't remember exactly what to do.
AHG will not change the banking on any track segment that you color pure black. Use this on camelback airtime hills. Color the crests, the negative g portion, pure black and their banking will remain zero.
Keep in mind you can utilize this technique to keep any banking you want on any elements, like turns and loops, etc. It doesn't necessarily have to be 0 banking either.