Filtering admin view of CPT by Relationship to another CPT

I'd like to filter the list of "episodes" in the admin edit view so that only episodes related to a selected "show" are returned. I set the filter up, and am passing the show ID in as a parameter, but the query is coming back with zero results.

Here's the code:

add_action( 'pre_get_posts', 'filter_episodes_by_selected_show' );
function filter_episodes_by_selected_show($query) {
    global $pagenow, $typenow;

    if ($typenow == 'episodes' && is_admin() && $pagenow == 'edit.php' && isset($_GET['filter_show']) && $_GET['filter_show'] != '') {
        $show_id = intval($_GET['filter_show']);
        
        // Query for episodes related to the selected show via MetaBox
        $meta_query = array(
            array(
                'key' => 'episodes-to-shows',
                'value' => $show_id,
                'compare' => '='
            )
        );

        $query->set('meta_query', $meta_query);
    }
}

What am I doing wrong? I've tried a few things aleady:

1. Testing on archive page (non-admin) using just the MB_Relationships_API::get_connected() function, but that's returning nothing as well.
2. Checking the relationship ID (multiple times... it works to display related info in the main app, but not through the query or API.
3. Using a WP_query() instead of the MB API

Is it a timing issue? Why would the API call fail? Is there another way to do this?

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