As you can see, you have anyRequest().authenticated(), that means, it restricts access to only authenticated users. Your / endpoint belongs to that anyRequest(), and that's why it's restricted to authenticated users and your request is not reaching to the method level. So whatever @preAuthorize you use, it's useless. You can check the javadoc here:

/*
 * Copyright 2004, 2005, 2006 Acegi Technology Pty Limited
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.security.authentication;

import org.springframework.security.core.Authentication;

/**
 * Evaluates <code>Authentication</code> tokens
 *
 * @author Ben Alex
 */
public interface AuthenticationTrustResolver {

    /**
     * Indicates whether the passed <code>Authentication</code> token represents an
     * anonymous user. Typically the framework will call this method if it is trying to
     * decide whether an <code>AccessDeniedException</code> should result in a final
     * rejection (i.e. as would be the case if the principal was non-anonymous/fully
     * authenticated) or direct the principal to attempt actual authentication (i.e. as
     * would be the case if the <code>Authentication</code> was merely anonymous).
     * @param authentication to test (may be <code>null</code> in which case the method
     * will always return <code>false</code>)
     * @return <code>true</code> the passed authentication token represented an anonymous
     * principal, <code>false</code> otherwise
     */
    boolean isAnonymous(Authentication authentication);

    /**
     * Indicates whether the passed <code>Authentication</code> token represents user that
     * has been remembered (i.e. not a user that has been fully authenticated).
     * <p>
     * The method is provided to assist with custom <code>AccessDecisionVoter</code>s and
     * the like that you might develop. Of course, you don't need to use this method
     * either and can develop your own "trust level" hierarchy instead.
     * @param authentication to test (may be <code>null</code> in which case the method
     * will always return <code>false</code>)
     * @return <code>true</code> the passed authentication token represented a principal
     * authenticated using a remember-me token, <code>false</code> otherwise
     */
    boolean isRememberMe(Authentication authentication);

    /**
     * Indicates whether the passed <code>Authentication</code> token represents a fully
     * authenticated user (that is, neither anonymous or remember-me). This is a
     * composition of <code>isAnonymous</code> and <code>isRememberMe</code>
     * implementation
     * <p>
     * @param authentication to test (may be <code>null</code> in which case the method
     * will always return <code>false</code>)
     * @return <code>true</code> the passed authentication token represented an
     * authenticated user ({@link #isAuthenticated(Authentication)} and not
     * {@link #isRememberMe(Authentication)}, <code>false</code> otherwise
     * @since 6.1
     */
    default boolean isFullyAuthenticated(Authentication authentication) {
        return isAuthenticated(authentication) && !isRememberMe(authentication);
    }

    /**
     * Checks if the {@link Authentication} is not null, authenticated, and not anonymous.
     * @param authentication the {@link Authentication} to check.
     * @return true if the {@link Authentication} is not null,
     * {@link #isAnonymous(Authentication)} returns false, &
     * {@link Authentication#isAuthenticated()} is true.
     * @since 6.1.7
     */
    default boolean isAuthenticated(Authentication authentication) {
        return authentication != null && authentication.isAuthenticated() && !isAnonymous(authentication);
    }

}

As you can see isAuthenticated(Authentication authentication) denies the anonymous user.

One thing you can do for achieving the anonymous restriction(to prevent authenticated users), you can add this:

.requestMatchers("/").anonymous()

By default, Spring Security's configuration redirects unauthorized requests to the login page for authentication. This behaviour you are facing is absolutely fine.