Java Reactive hangs when calling repo operation inside map

Question

I'm new to this reactive world and having an issue when I want to map my user to another user.

So the code is supposed to go to DB to find the user based on Username, then will need to create another user object and populate its Authority by calling another repository.

Then I need the value so I used block.

Mono<User> userMono = userService.findByUsername(regularUser.getUsername())
            .map(u -> {
                      User user = new User();
                      user.setUsername(u.getUsername());
                      user.setAuthority(userService.getAuthorities(u));
                      return user;
            });
User usr = userMono.block();

I found that every time the code executes the block, it will stop working. the request to DB is never completed. I debugged and found that in Mono#block - BlockingSingleSubscriber#blockingGet the getCount() is 1, which will go to await(). This takes forever. I waited but the getCount() didn't reach 0 and there was no InterruptedException thrown. blockingGet

UserService.java

@Autowired
UserRepository userRepository;

@Autowired
AuthorityRepository authorityRepository;

public Mono<User> findByUsername(String username) {
  return userRepository.findByUsername(username);
}
public Flux<Authority> getAuthorities(User user) {
  return authorityRepository.findAllByUserId(user.getId());
}

I'm using Spring Boot 2.7.0
io.spring.dependency-management 1.0.11.RELEASE
spring-boot-starter-webflux
spring-boot-starter-data-mongodb-reactive

Reactive UserDetailService bean that has the problem with calling repository inside a map

@Bean
public PasswordEncoder passwordEncoder() {
    return PasswordEncoderFactories.createDelegatingPasswordEncoder();
}

@Bean
public ReactiveAuthenticationManager reactiveAuthenticationManager(ReactiveUserDetailsService userDetailsService,
                                                                   PasswordEncoder passwordEncoder) {
    var authenticationManager = new UserDetailsRepositoryReactiveAuthenticationManager(userDetailsService);
    authenticationManager.setPasswordEncoder(passwordEncoder);
    return authenticationManager;
}

@Bean
public ReactiveUserDetailsService userDetailsService(UserService userService) {

    return username -> userService.findByUsername(username)
            .log()
            .map(u -> org.springframework.security.core.userdetails.User
                    .withUsername(u.getUsername()).password(u.getPassword())                                .roles(userService.getRoles(u).toArray(String[]::new))
                    .accountExpired(!u.isActive())
                    .credentialsExpired(!u.isActive())
                    .disabled(!u.isActive())
                    .accountLocked(!u.isActive())
                    .build()
            );
}

If I hardcoded the roles (without going to the database) it works well.

I created a simple project that replicate the issue: https://github.com/skyglassio/repository-await/blob/master/src/main/java/com/example/demo/DevInitializer.java line 57. After userMono.block(); is called it goes into a forever wait.