OnePlus 9 and OnePlus 9 Pro users are facing low-storage issues apparently due to a preloaded app called Media Storage. Some users have also reported that the app is causing rapid battery drain and is slowing down their phones.

source https://gadgets.ndtv.com/mobiles/news/oneplus-9-pro-media-storage-app-issues-problems-bug-fix-slowing-down-battery-drain-2517436#rss-gadgets-all