Is it possible to retrieve all posts with a certain value for metadata?

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”

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