Output Meta Description with Custom Field

Hello!

What you needed to change was fl_description to prop_description.

add_filter( 'the_seo_framework_fetched_description_excerpt', function( $excerpt, $depr, $args ) {

    if ( null === $args ) {
        // In the loop.
        $get_field = is_singular();
    } else {
        // Out the loop; only $args['id'] may be filled...
        $get_field = $args['id'] && empty( $args['taxonomy'] ) && empty( $args['pta'] );
    }

    $field = '';

    if ( $get_field ) {
        $field = function_exists( 'get_field' ) ? get_field( 'prop_description', $args['id'] ?? null ) : '';
        $field = $field ? tsf()->s_excerpt_raw( $field ) : '';
    }

    return $field ?: $excerpt;
}, 10, 3 );

I did that for you in the snippet above ?? Cheers!

(I also used tsf() instead of the_seo_framework(), available since TSF v4.2+)