经过长时间的挣扎，我找到了解决办法。

@Override
protected KeycloakAuthenticationProvider keycloakAuthenticationProvider() {
    return new KeycloakAuthenticationProvider() {

        @Override
        public Authentication authenticate(Authentication authentication) throws AuthenticationException {
            KeycloakAuthenticationToken token = (KeycloakAuthenticationToken) authentication;
            List<GrantedAuthority> grantedAuthorities = new ArrayList<>();

            for (String role : ((KeycloakPrincipal) token.getPrincipal()).getKeycloakSecurityContext().getToken().getRealmAccess().getRoles()) {
                grantedAuthorities.add(new KeycloakRole(role));
            }

            return new KeycloakAuthenticationToken(token.getAccount(), token.isInteractive(), new SimpleAuthorityMapper().mapAuthorities(grantedAuthorities));
        }

    };
}

我们可以通过在Security配置类中使用以下方法将角色设置为KeycloakAuthenticationToken来实现解决方案。