Increase Custom Fields Dropdown Limit
<!-- For your functions.php file or custom plugin: -->
// Increase number of meta fields shown in dropdown
add_filter('postmeta_form_limit', 'customfield_limit_increase');
function customfield_limit_increase($limit) {
$limit = 100;
return $limit;
}
<!-- Otherwise, the dropdown for Custom Fields is capped at 30 (alphabetically). -->