I am assuming your custom post type is also “series”.
Can you try this, it should work.
$args = [
'post_type' => 'series',
'posts_per_page' => -1,
'meta_query' => [
[
'key' => 'series',
'value' => 19,
'compare' => '=',
'type'=>'NUMERIC'
]
],
];
$posts = new WP_Query( $args );
or you can simple use:
$args = [
'post_type' => 'series',
'posts_per_page' => -1,
'meta_key'=>'series',
'meta_value'=>19,
'meta_compare'=>'=',
'meta_type'=>'NUMERIC'
];
$posts = new WP_Query( $args );
If you are using these meta values as NUMERIC it will be essential to provide “meta_type”, as WordPress treats “Numeric” and “Char” values very differently. As you seems to use these value as Number, you need to provide “meta_type”