There is no ad in the app. it’s a bug. If you want to re-create it. Follow a link to someplace like a GitHub project. You’ll notice a mobile ad banner at the top of the screen. Now go back to the voyager pwa interface. If you’re at the top and there are no other posts off of the top of the screen. Slowly swipe left or right and pull down. It’ll bring the banner from the previous page you visited into view.