Why does my FileWriter add some data to its own line and other data to ends of other lines?

Question

I am making an application to act as a simple classroom library with books and users. There are teacher and student user types. I have no database and understand that the program can't run at all times, so the changed/added information while it is running should be saved when the program ends. The problem I am facing is that it is writing the information to the csv file in a manner I don't understand.

Main.java File

public static void main(String[] args) throws IOException {
        Library library = new Library();
        User teacher = new Teacher("admin", "password");
        library.addUser(teacher);
        library.loadUsers("/path/to/users.csv");
        library.addUser(new Student("student3"));
        //library.loadBooks("/path/to/books.csv");
        //library.loginTeacher("admin", "password");
        
        //library.addBook(new Book("hhg", "hhg", "hjhjf", 40));
        //library.printBooks();
        //library.saveBooks("/path/to/books.csv");

        library.saveUsers("/path/to/users.csv");

    }

users.csv

student1,
student2,

Library.java

public void loadUsers(String userDir) {

        File myFile1 = new File(userDir);
        Scanner scanner = null;
        try {
            scanner = new Scanner(myFile1);
            scanner.useDelimiter(",");
        } catch (FileNotFoundException e) {
            System.out.println("file not found");
            e.printStackTrace();
        }

        ArrayList<String[]> data = new ArrayList<>();

        while (scanner.hasNext()) {

            String line = scanner.next();

            String[] r = line.split(",");
            data.add(r);
        }

        for(int i = 0; i < data.size(); i++) {
            User user = new Student(data.get(i)[0]);
            addUser(user);
        }
}

public void saveUsers(String usersDir) throws IOException {
        File myFile = new File(usersDir);

        FileWriter fw = null;
        try {
            fw = new FileWriter(myFile);
        } catch (IOException e) {
            e.printStackTrace();
        }
        
        for(int i = 0; i < users.size(); i++){
            if(users.get(i) != null) {
                try {
                    if(users.get(i) instanceof Student) {
                        fw.write(String.format("%s,", users.get(i).getUsername()));
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        fw.close();
    }

Resulting users.csv

student1,
student2,student3,