get_post_meta is always empty when I use wp_mail

I made this action hook which will send an email to the users every time a new post in the custom post type “events” is made.

function email_members($post_id) {
global $wpdb;

$usersarray = $wpdb->get_col("SELECT user_email FROM wp_users");
$users = implode(",", $usersarray);

if( ( $_POST['post_status'] == 'publish' ) && ( $_POST['original_post_status'] != 'publish' ) ) {
    $subject = 'Event Reminder';

    $post_url = get_permalink($post_id);
    $post_title = get_the_title($post_id);
    $event_date = get_post_meta( $post_id, 'ch_event_date', true );

    $message = "APAC Ministries Event Reminder for: ".$post_title."\n\n";
    $message .= "Date: ".$event_date;

    wp_mail($users, $subject, $message );
  }
}

add_action('publish_th_events', 'email_members');

However, the $event_date is always empty. I tried running this code in a different file and it works:

global $post;
$test = get_post_meta($post->ID, 'ch_event_date', true);
echo $test;

I can’t seem to use get_post_meta in wp_mail.

Any alternative solutions are welcome. Thanks!