Why is functions.php file in child not initializing and over-writing parent?

so James in order to achieve your goal you need to remove the meta-box and re-add it with the new callback that contain the new value;

so lets assume originally you have the meta-box added as below :

function adifier_custom_meta()
{
    $screens = ['page', 'post'];
    foreach ($screens as $screen) {
        add_meta_box(
            'advert_cond', // Unique ID
            esc_html__('Condition', 'adifier'), // Box title
            'adifier_get_advert_meta', // Content callback, must be of type callable
            $screen, // Post type,
            'side'
        );
    }
}
add_action('add_meta_boxes', 'adifier_custom_meta');

and the original call-back output as following :

function adifier_get_advert_meta($post)
{
    $value = get_post_meta($post->ID, '_advert_cond_meta_key', true);
    ?>
    <label for="advert_cond">Description for this field</label>
    <select name="advert_cond" id="advert_cond" class="postbox">
        <option value="0"><?php echo esc_html__('-Select-', 'adifier') ?></option>
        <option value="1" <?php selected($value, '1');?>><?php echo esc_html__('New with tags', 'adifier') ?></option>
        <option value="1" <?php selected($value, '2');?>><?php echo esc_html__('New with tags', 'adifier') ?></option>
        <option value="3" <?php selected($value, '3');?>><?php echo esc_html__('Used with tags', 'adifier') ?></option>
        <option value="4" <?php selected($value, '4');?>><?php echo esc_html__('For Parts Or Not Working', 'adifier') ?></option>

    </select>
    <?php
}

in order to change above we need to add the following functions:

add_action('add_meta_boxes', 'adifier_custom_meta2');
function adifier_custom_meta2()
{
    $screens = ['page', 'post'];
    remove_meta_box('advert_cond', $screens, 'side');
    add_meta_box(
        'advert_cond', // Unique ID
        esc_html__('Condition', 'adifier'), // Box title
        'adifier_get_advert_meta2', // Content callback, must be of type callable
        $screens, // Post type,
        'side'
    );
}

and now we will add our new call-back output with our new values :

function adifier_get_advert_meta2($post)
{
    $value = get_post_meta($post->ID, '_advert_cond_meta_key', true);
    ?>
    <label for="advert_cond">Description for this field</label>
    <select name="advert_cond" id="advert_cond" class="postbox">
        <option value="0"><?php echo esc_html__('-Select-', 'adifier') ?></option>
        <option value="1" <?php selected($value, '1');?>><?php echo esc_html__('New', 'adifier') ?></option>
        <option value="2" <?php selected($value, '3');?>><?php echo esc_html__('Used', 'adifier') ?></option>
        <option value="3" <?php selected($value, '4');?>><?php echo esc_html__('For Parts Or Not Working', 'adifier') ?></option>

    </select>
    <?php
}

for more information you can check WordPress Doc from the following links:

Custom Meta Boxes

Add Meta Box