Wordpress admin filter by meta, filter options disappear after search

Question

I wrote these functions in order to filter posts based on meta data. The context is a real estate site, with properties being a CPT. In these functions, I am filtering the properties on the admin side by the agents selling them. The function works on any fresh attempt. If an agent is selected, the filter will only show properties sold by that agent.

The problem I'm having persists after that initial filtering. The list of agents simply disappears. I'm getting the feeling that the loop I ran to create the list is being halted for whatever reason.

To visually clarify, here's an image of how the filter list looks before a search.

And here is how it looks after using the filter

Apparently if ADMIN_FILTER_FIELD_VALUE=(id number here) appears in the url with any set value, the loop won't run.

Here's the code for all of this.

add_filter( 'parse_query', 'agents_posts_filter' );
function agents_posts_filter( $query ){
    global $pagenow;
    $type = 'post';
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }
    if ( 'properties' == $type && is_admin() && $pagenow=='edit.php' && isset($_GET['ADMIN_FILTER_FIELD_VALUE']) && $_GET['ADMIN_FILTER_FIELD_VALUE'] != '') {
        $query->query_vars['meta_key'] = 'select-agent-value';
        $query->query_vars['meta_value'] = $_GET['ADMIN_FILTER_FIELD_VALUE'];
    }
}

add_action('restrict_manage_posts', 'filter_post_type_by_agent');
function filter_post_type_by_agent(){
    global $pagenow;
    $type = 'post';
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }

    if ('properties' == $type && is_admin() && $pagenow=='edit.php') {
        ?>
        <select name="ADMIN_FILTER_FIELD_VALUE">
        <option value=""><?php _e('Filter By Agent'); ?></option>
        <?php
            $args = array(
                'post_type' => 'agents',
                'posts_per_page' => -1                      
            );

            $posts = new WP_Query($args);

            if( $posts->have_posts() ): while( $posts->have_posts() ) : $posts->the_post(); ?>

                <option value="<?php the_ID(); ?>"> <?php the_title(); ?> </option>

            <?php

            endwhile; 

            endif; 
        ?>
        </select>
        <?php
    }
}

Is there something obvious I'm missing here? Thanks for any help anyone can provide.

Answer 1

I think the query_vars (meta key and meta value) also add into query WP_Query post type agents below. (Dump $posts to check meta_query) Try replacing WP_Query with get_posts. Maybe help! ^^

EDIT:

The above query also add into below query. So I try and fix it below.

add_action( 'pre_get_posts', 'agents_posts_filter' );
function agents_posts_filter( $query ){
    global $pagenow;
    $type = 'post';
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }
    if ( 'post' == $type && is_admin() && $pagenow=='edit.php' && isset($_GET['abc']) && $_GET['abc'] != '' && $query->is_main_query()) {
        $query->set('meta_key', 'select-agent-value');
        $query->set('meta_value', $_GET['abc']);
    }
}