@MockBean service field is always null in a Test class with @WebMvcTest

Question

I'm trying to improve a unit test for a service I'm developing. I read somewhere that it's ideal to use @WebMvcTest annotation over @SpringBootTest when testing the Web or Controller layer.

However, for some reason, a @MockBean service field I am using in the Test class is always NULL.

java.lang.AssertionError: Expecting actual not to be null

In the test class below, serviceIsLoaded() method is always null when I run that single test.

SectionRESTControllerTest.java

@WebMvcTest
public class SectionRESTControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private SectionServiceImpl sectionServiceImpl;

    @Test
    public void serviceIsLoaded() {
        assertThat(sectionServiceImpl).isNotNull();
    }
}

SectionServiceImpl.java

@Service
public class SectionServiceImpl implements SectionService {

    private final Logger logger = LoggerFactory.getLogger(SectionServiceImpl.class);
    @Autowired
    private SectionRepository sectionRepo; //saves to section table
    @Autowired
    private GradeLevelRepository gradeLevelRepo;

    @Override
    public Section createSection(Section section) {...}

    @Override
    public Section updateSection(Section request) throws Exception {...}

    @Override
    public Section getSectionById(Long id) {
        return sectionRepo.findById(id).get();
    }

    @Override
    public List<Section> getAllActiveSections() {
        return sectionRepo.findSectionByIsActiveTrue();
    }

    @Override
    public List<Section> getAllInActiveSections() {
        return sectionRepo.findSectionByIsActiveFalse();
    }

    @Override
    public List<Section> getSectionsByGradeLevelId(Long id) {
        return sectionRepo.findByGradeLevelId(id);
    }

    @Override
    public List<Section> getSectionByGradeLevelCode(String code) {
        return sectionRepo.findByGradeLevelCode(code);
    }

    @Override
    public List<Section> getSectionByGradeLevelCategory(String category) {
        return sectionRepo.findByGradeLevelCategory(category);
    }
}

My understanding is, with @WebMvcTest, it does not load the entire application context with all managed beans which makes the UnitTest run faster. Unlike, @SpringBootTest which loads everything when running the UnitTest.

SectionService.java

public interface SectionService {

    Section createSection(Section section);
    Section updateSection(Section section) throws Exception;
    Section getSectionById(Long id);
    List<Section> getAllActiveSections();
    List<Section> getAllInActiveSections();

    List<Section> getSectionsByGradeLevelId(Long id);
    List<Section> getSectionByGradeLevelCode(String code);
    List<Section> getSectionByGradeLevelCategory(String category);

}

Main class

@SpringBootApplication
public class AutoformSettingsServiceApplication {

    public static void main(String[] args) {
        SpringApplication.run(AutoformSettingsServiceApplication.class, args);
    }

}

Do you have any thoughts or ideas why the @MockBean sectionServiceImpl is null

Thank you.