get_post_meta returns NULL in front-end, but correct result in back-end

Not 100% sure I understand the question or problem, but this ought to work, I think:

add_action( 'the_post', 'output_my_custom_field');

function output_my_custom_field( $post_object ) {

    $post_id = $post_object->ID;

    // why the conditional (and redundant !empty) ?
    // if ( !empty($post_id) ) ) { 

        $my_custom_field = get_post_meta( $post_id, 'my_custom_field', true );

        var_dump($my_custom_field);

    // see above
    // }  

 }