Not every app is optimized to use the Galaxy S9's taller 18:9 display. Many apps are still optimized for the regular 16:9 aspect ratio that most smartphones used before 2016, and some apps are slightly cut off at the bottom as a result.
That doesn't stop Samsung from making sure you get the full screen experience. Head to Settings > Display > Full screen apps. There, you'll see a long list of apps that are already optimized for taller 18:9 aspect ratios. Scroll further down and you'll find the apps that aren't optimized. Some might already be enabled if you enabled ful- screen mode when you first open the apps, others might not if you missed that option.
Alternatively, you can just make sure to tap the bottom of the screen when the phone asks you if you want the app to fill the entire screen the next time you open it.