How can I write a function that would update any missing specific post metadata?

Try this, your code may be returning an empty string or false, then comparing it to another empty string or false thus returning true.

function check_post_update_meta ($post_id, $key, $value) {

   // response can receive post meta array, or '' or false
   $response = get_post_meta($post_id, $key, true);

    // isset checks for data, so in case $response is empty if() will not evaluate true
    if( isset($response) ){

        if ($response == $value) {

            return true;

        } else {

            $response = update_post_meta($post_id, $key, $value);
        }
    }
}