Why this mocked StringBuilder always return the string "stringBuilder" ? Posibly confiusion with Junit mocks

Question

Hello and sorry for my english. Im new in Junit and im trying to learning mocks and how it works, well, I have this test class with the next mocks:

    @DataJpaTest
    @AutoConfigureTestDatabase(connection = EmbeddedDatabaseConnection.H2)
    @ExtendWith(MockitoExtension.class)
    public class UserRepositoryTest {
        @Autowired
        private UserRepository userRepository;
        @Mock
        private SecureRandom secureRandom;
        @Mock
        private StringBuilder stringBuilder;
        @InjectMocks
        private StringUtilsApp stringUtilsApp;

As you can see im using StringUtilsApp, is a dependency and I inyect 2 mocks in it, This is the method im using in my test:

    @Component
    @RequiredArgsConstructor
    public class StringUtilsApp {
        private final SecureRandom secureRandom;
        private final StringBuilder stringBuilder;
    
        /**
         * Generates a random String
         *
         * @param length     the length of the String
         * @param characters the posibles characters of the String
         * @return a random String
         */
        public String randomStringGenerator(int length, String characters) {
            String charactersToUse = (characters != null && !characters.isEmpty()) ? characters : "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
            for (int i = 0; i < length; i++) {
                int randomIndex = secureRandom.nextInt(charactersToUse.length());
                char randomChar = charactersToUse.charAt(randomIndex);
                stringBuilder.append(randomChar);
            }
            return stringBuilder.toString();
        }
    }

This is the method where im using the randomStringGenerator in my JUNIT test:

    private User getARandomUser() {
        return User.builder()
                .username(stringUtilsApp.randomStringGenerator(80,null))
               .password(stringUtilsApp.randomStringGenerator(200,null))
                .email(stringUtilsApp.randomStringGenerator(200, null))
                .role(Role.ADMIN)
                .build();
    }

The bean of StringBuilder that I inyect in the class StringUtils :

    @Configuration
    @RequiredArgsConstructor
    public class ApplicationConfig {
        private final UserRepository userRepository;
    
    
        @Bean
        @Scope(value = ConfigurableBeanFactory.SCOPE_PROTOTYPE)
        public StringBuilder stringBuilder() {
            return new StringBuilder();
        }
    }

Well with all this context when I use my method randomStringGenerator in any method of my test the StringBuilder always return the string "stringBuilder" and only happend in my tests, if I use in other class of the app inyecting the dependency, the returning string is the random String that i wait:

Result using the method in any Junit test

Why is happend this? Is there something Im not taking into account? Thanks. PD: Any corrections or other good practices about my code are wellcoming :)

Im wating to know what is happening with this bean and whats the problem with the mock.