Even if the app itself is rated for mature audiences, everything visible on the public App Store listing — name, icon, screenshots, description — has to be suitable for all ages, since anyone browsing the Store can see it.
Common with mature-content or dating apps that use suggestive screenshots or explicit language in the public description, even when the in-app content itself is properly age-gated.
"Your app's metadata includes content that is not appropriate for all audiences, per guideline 2.3.8, regardless of your app's content or age rating."