Cannot mock method beavior on invocation inside configuration class

In my Spring Boot project I have a configuration class like this:

@Configuration
@RequiredArgsConstructor // ... using Lombok
public class MyConfigClass {
    private final MyService myService;
    
    
    @PostContruct
    void setup(){
        String s = myService.aMethod();
        System.out.println(s.substring(0, 2));
    }
}

Now I need to test some features of my project and for these tests I need to mock MyService class:

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.NONE)
class SomeOtherServiceTest {
    @MockBean
    private MyService mockMyService;

    @BeforeEach
    void setup() {
        when(mockMyService.aMethod()).thenReturn("blablabla"); // ... using Mockito
    }

    @Test
    void test() {
        // test code
    }
}

When I run test I’m getting a NullPointerException on s.substring(0, 2) invocation in my configuration class. This means that the method has not been yet mocked at this point.

How can I achieve this?

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