Before migrating to Spring Boot 2.1, we had a couple of controller tests in our services utilizing @WebMvcTest in combination with @AutoConfigureMockMvc
Encountered the same scenario and what helped was using the below annotations instead of @WebMvcTest. In this case, @WithMockUser did not help.
@WebAppConfiguration
@Import({MockMvcAutoConfiguration.class})
@EnableConfigurationProperties({ResourceProperties.class, WebMvcProperties.class})
Classes that existed in controllers / value of @WebMvcTest goes into value of @Import annotation.
Source: https://github.com/spring-projects/spring-boot/issues/14227#issuecomment-688824627