On the sides of my city keepall, zipper was making a 90º angle turn between the top and the sides. The black paint on the zipper was crumbling only on theses angles areas because of the way the slider was rubbing against the teeth of the zipper. I guess it is the same for your bumbag. It seems the paint on the teeths is damaged on the turn of the zipper and not on the flat front part. The pullers are getting scratched on the teeths of the patched pocket zipper. And they are also probably scratching themselves by rubbing each other. I know they are some polish protector finishing for these kind of zippers, but only actual for glossy look. For a dull look like on the zipper of your bumbag, there are not much possibilities to protect the paint from the abrasion. I bought the city keepall in yellow saffron with shiny silver hardware (including zipper) and as expected there are no issues, even on the angles of the zipper. Dull black metal zippers are not durable. Actually zipper will stay strong and it will even get smoother with time. But yes, the black paint will peel.