Spring Security keeps redirecting me to formLogin page

Question

Spring Security keeps redirecting me to formLogin page after any exception is thrown. To add more context to issue: To register a new user the http://localhost:8080/api/v1/registration/register end point is called. the first time its called it adds the user successfully no problem. But if i hit that API again with same details of user i just created, then the "email already taken" error is thrown in the server logs, as it should. But instead of simply showing error, the program tried to redirect to /login endpoint when error was thrown. This is my websecurityconfig settings:

package com.example.venuematchingservice.security.config;

import com.example.venuematchingservice.appuser.AppUserService;
import lombok.AllArgsConstructor;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.security.authentication.dao.DaoAuthenticationProvider;
import org.springframework.security.config.Customizer;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityCustomizer;
import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
import org.springframework.security.web.SecurityFilterChain;


@Configuration
@AllArgsConstructor
@EnableWebSecurity
public class WebSecurityConfig {

    private final AppUserService appUserService;
    private final BCryptPasswordEncoder bCryptPasswordEncoder;

    @Bean
    public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
        http
                .csrf(csrf -> csrf
                        .disable())
                .authorizeHttpRequests(auth -> auth
                        .requestMatchers("/api/v1/registration/**", "*").permitAll()
                        .anyRequest().authenticated())
                .formLogin(Customizer.withDefaults());
        return http.build();
    }

    @Bean
    public WebSecurityCustomizer webSecurityCustomizer() {
        return (web) -> web.ignoring().requestMatchers("/js/**", "/images/**");
    }


    @Bean
    public DaoAuthenticationProvider authenticationProvider() {
        DaoAuthenticationProvider authProvider = new DaoAuthenticationProvider();
        authProvider.setUserDetailsService(appUserService);
        authProvider.setPasswordEncoder(bCryptPasswordEncoder);
        return authProvider;
    }
}

and here is the class throwing the error:

package com.example.venuematchingservice.appuser;

import com.example.venuematchingservice.registration.token.ConfirmationToken;
import com.example.venuematchingservice.registration.token.ConfirmationTokenService;
import lombok.AllArgsConstructor;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UserDetailsService;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
import org.springframework.stereotype.Service;

import java.time.LocalDateTime;
import java.util.UUID;

@Service
@AllArgsConstructor
public class AppUserService implements UserDetailsService {

    private final AppUserRepository appUserRepository;
    private final static String USER_NOT_FOUND = "user with email %s not found";
    private final BCryptPasswordEncoder bCryptPasswordEncoder;
    private ConfirmationTokenService confirmationTokenService;

    @Override
    public UserDetails loadUserByUsername(String email) throws UsernameNotFoundException {
        return appUserRepository.findByEmail(email).orElseThrow(() ->  new UsernameNotFoundException(String.format(USER_NOT_FOUND, email)));
    }

    public String signUpUser(AppUser appUser){
        boolean userExists = appUserRepository.findByEmail(appUser.getEmail()).isPresent();

        if (userExists){
            throw new IllegalStateException("email already taken");
        }

        String encodedPassword = bCryptPasswordEncoder.encode(appUser.getPassword());
        appUser.setPassword(encodedPassword);
        appUserRepository.save(appUser);

        String token = UUID.randomUUID().toString();
        ConfirmationToken confirmationToken = new ConfirmationToken(
                token,
                LocalDateTime.now(),
                LocalDateTime.now().plusMinutes(15),
                appUser
        );
        confirmationTokenService.saveConfirmationToken(confirmationToken);

        return token;
    }

    public int enableAppUser(String email) {
        return appUserRepository.enableAppUser(email);
    }
}

Any ideas why throwing errors would cause a redirect to login page?

I've tried changing WebSecurityConfigs and Custom error handeling but nothing worked