Android 12 introduces a new "SplashScreen" API:

If you have previously implemented a custom splash screen in Android 11 or lower, you’ll need to migrate your app to the SplashScreen API to ensure that it displays correctly starting in Android 12. Not migrating your app will result in a degraded or unintended app launch experience.

Currently, Qt uses a custom implementation to show the splash screen. When running a Qt-based app using a splash screen on Android 12 this results in a broken launch experience:

1. First the app icon is shown as a splash screen
2. Then the actual splash screen is shown

This article contains the steps that need to be done for the migration:

https://developer.android.com/guide/topics/ui/splash-screen/migrate