In Android 9 on Qt 5.11.2 and using API 28 (tried others as well), at startup, I'm seeing output like this on simplistic Qt apps:
Accessing hidden method Landroid/graphics/drawable/BitmapDrawable;->getTintMode()Landroid/graphics/PorterDuff$Mode; (light greylist, reflection)
java.lang.NoSuchFieldException: No field mPivotX in class Landroid/graphics/drawable/RotateDrawable$RotateState;
When you look into this you discover that Android is going start disabling applications that use private "non-SDK" interfaces: https://developer.android.com/about/versions/pie/restrictions-non-sdk-interfaces
You can suppress the warning popup by using an SDK target of 28, but, that doesn't change the new policy being pushed by Google to stop all applications (including all those written using Qt) from accessing non-public APIs.