meta-box-tsf-post-description.php

<?php
// Don't include the PHP opening tag if PHP is already open.

// Previous: https://gist.github.com/sybrew/299ad19597f974c89b1564316297c1ed
// For: https://wordpress.org/support/topic/fetch-meta-description-on-custom-field-from-meta-box/

add_filter( 'the_seo_framework_generated_description', 'my_tsf_generated_description', 10, 2 );
/**
 * @param string     $desc The generated description.
 * @param array|null $args The query arguments. Contains 'id', 'tax', 'pta', and 'uid'.
 *                         Is null when the query is auto-determined.
 * @return string The overwritten description.
 */
function my_tsf_generated_description( $desc, $args ) {

	// If Meta Box isn't activated, don't do anything.
	if ( ! function_exists( 'rwmb_get_value' ) ) return $desc;

	if ( isset( $args ) ) {
		// Admin area.
		switch ( The_SEO_Framework\get_query_type_from_args( $args ) ) {
			case 'single':
				$post_id = $args['id'];
		}
	} else {
		// On the front-end.
		$tsfquery = tsf()->query();

		if ( $tsfquery->is_singular() ) {
			$post_id = $tsfquery->get_the_real_id();
		}
	}

	if ( ! empty( $post_id ) ) 
		$desc = wp_strip_all_tags( rwmb_get_value( 'short_description', [], $post_id ) ) ?: $desc;
	
	return $desc;
}