Custom Field select list is truncated

As explained here there is a WordPress limit of 30 for the number of custom fields that are available by default. But you can change the behavior as shown with the hook postmeta_form_limit in the below example.

add_filter( 'postmeta_form_limit' , 'customfield_limit_increase' );
function customfield_limit_increase( $limit ) {
    $limit = 100;
    return $limit;
}