ACF/WordPress action to help with sorting serialized custom fields by meta_value

make-shadow-metadata.php

<?php

/**
 * ACF's serialized data can't be used for sorting posts,
 * so we're making a more useful copy for the purpose.
 *
 * @param int|string $post_id The ID of the post being edited.
 * @return void
 */
function happyhumans_save_plaintext_metadata( $post_id ) {
	// Get newly saved values.
	$values = get_fields( $post_id );

	// If this post has a "state" field, parse it down to plain-text and save it as a shadow record.
	if ( ! empty( $values['state'] ) ) {
		$plain_text_state = '';
		foreach ( $values['state'] as $state ) {
			$plain_text_state .= $state . ' ';
		}
		update_post_meta( $post_id, 'plain_text_state', trim( strtolower( $plain_text_state ) ) );
	}
}

add_action( 'acf/save_post', 'happyhumans_save_plaintext_metadata' );

The full explanation is here.