Dart if(A is B) condition not working properly - flutter

Question

var data = snapshot.data as List;
return ListView.builder(
itemCount: data.length,
itemBuilder: (context, i) {
var item = data[i];
print('instance: $item');
if (item is BookFavorItem) {
return BookItemCard(
bookId: item.bookId,
title: item.title,
author: item.author,
);
} else if (item is BookLentItem) {
return BookItemCard(
bookId: item.bookId,
title: item.title,
author: item.author,
dateRange: '${item.lentDate} - ${item.deadline}',
);
} else {
return ListTile(
title: Text('<$item> not matched'),
);
}
},
);
Console Output
I/flutter ( 6832): instance: Instance of 'BookFavorItem'
I/flutter ( 6832): instance: Instance of 'BookFavorItem'
I/flutter ( 6832): instance: Instance of 'BookFavorItem'
I/chatty ( 6832): uid=10336(com.example.library) 1.ui identical 6 lines
I/flutter ( 6832): instance: Instance of 'BookFavorItem'
I/flutter ( 6832): instance: Instance of 'BookFavorItem'
I/chatty ( 6832): uid=10336(com.example.library) 1.ui identical 5 lines
I/flutter ( 6832): instance: Instance of 'BookFavorItem'
Screen Result
As you can see from the code, I got a variable called item which is an instance of BookFavorItem.
However, when I use condition if( item is BookFavorItem ) to control the flow, it did not work as expected behavior. (I thought this condition should be true, but it didn't)
The evidence is that I got the message of "Instance of BookFavorItem", which is the exact type used for is condition from Debug Console and App Screen
What's wrong with me?
-> flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.17.1, on Microsoft Windows [Version 10.0.19041.264], locale zh-CN)
[!] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
X Android license status unknown.
Try re-installing or updating your Android SDK Manager.
See https://developer.android.com/studio/#downloads or visit visit
https://flutter.dev/docs/get-started/install/windows#android-setup for detailed instructions.
[!] Android Studio (version 3.6)
X Flutter plugin not installed; this adds Flutter specific functionality.
X Dart plugin not installed; this adds Dart specific functionality.
[√] VS Code (version 1.45.1)
[√] Connected device (1 available)