Overall Problem: ACF fields in front end registration not saving to user profile

Having some trouble with front end registration and ACF. I've got ACF fields attached to a custom user role, but I'm unable to save them through front end registration

First, my form.

function custom_registration_form_fields() {

    ob_start(); ?>
        <h3 class="custom_header"><?php _e('Register New Account'); ?></h3>

        <?php custom_show_error_messages(); ?>

        <form id="custom_registration_form" class="custom_form" action="" method="POST">
            <fieldset>
                <p>
                    <label for="custom_user_Login"><?php _e('Username'); ?></label>
                    <input name="custom_user_login" id="custom_user_login" class="required" type="text"/>
                </p>
                <p>
                    <label for="custom_user_email"><?php _e('Email'); ?></label>
                    <input name="custom_user_email" id="custom_user_email" class="required" type="email"/>
                </p>
                <p>
                    <label for="custom_user_first"><?php _e('First Name'); ?></label>
                    <input name="custom_user_first" id="custom_user_first" type="text"/>
                </p>
                <p>
                    <label for="custom_user_last"><?php _e('Last Name'); ?></label>
                    <input name="custom_user_last" id="custom_user_last" type="text"/>
                </p>
                <p>
                    <label for="password"><?php _e('Password'); ?></label>
                    <input name="custom_user_pass" id="password" class="required" type="password"/>
                </p>
                <p>
                    <label for="password_again"><?php _e('Password Again'); ?></label>
                    <input name="custom_user_pass_confirm" id="password_again" class="required" type="password"/>
                </p>
                <?php custom_acf_registration_fields();?>
                <p>
                    <input type="hidden" name="custom_register_nonce" value="<?php echo wp_create_nonce('custom-register-nonce'); ?>"/>
                    <input type="submit" value="<?php _e('Register Your Account'); ?>"/>
                </p>

            </fieldset>
        </form>
    <?php
    return ob_get_clean();
  }

And here's the function adding my ACF form fields.

function custom_acf_registration_fields()
{
    if(function_exists('acf_form'))
    {
        if ( is_user_logged_in() === true )
        {
            global $current_user;
            $user_id = 'user_'.$current_user->ID;
        }
        else
            $user_id = 'user_new';

        $args = array(
            'post_id' => $user_id,
            'field_groups' => array(141),
            'form' => false
        );

        acf_form( $args );
    }
}

add_action('register_form', 'custom_acf_registration_fields');

And then I've attached this to the header.

function add_acf_form_head(){
    global $post;

  if ( !empty($post) && has_shortcode( $post->post_content, 'register_form' ) ) {
        acf_form_head();
    }
}
add_action( 'get_header', 'add_acf_form_head' );

And lastly, I have my save ACF values.

    function save_additional_user_meta( $user_id ) {

    if ( isset( $_POST['fields']['field_5e4acd8bb31a0'] ) ) {
        update_user_meta($user_id, 'country', $_POST['fields']['field_5e4acd8bb31a0']);
    }

    if ( isset( $_POST['fields']['field_5e4ace947d640'] ) ) {
        update_user_meta($user_id, 'tagline', $_POST['fields']['field_5e4ace947d640']);
    }

    if ( isset( $_POST['fields']['field_5e4ace3db31a1'] ) ) {
        update_user_meta($user_id, 'profile_image', $_POST['fields']['field_5e4ace3db31a1']);
    }
}

add_action( 'user_register', 'save_additional_user_meta', 10, 1 );

Unfortunately, this is not saving my ACF fields to the user profile, and I'm not sure why. It's also not saving the user with the custom user role I made, despite setting it in the admin config.