Hey,
I really didn’t get the question, but rather than filtering it in the query, you can use this.
$posts = get_posts(array(
'post_type' => 'letters',
'numberposts' => -1,
));
foreach($posts as $post)
{
if(get_field('letter_sent', $post->ID) == '2019')
{
//your code
}
}
Also the meta_key is not same when acf saves it. Try enabling the Custom fields option in the Screen options. You will get the idea how acf saves the custom fields.
