Increase the number of custom fields to retrieve for the drop-down in the Custom Fields meta box.

gistfile1.txt

#Custom Fields Dropdown - WordPress Admin Dashboard

By default limit is 30 keys shown in the dropdown. If you want to increase the limit.

add_filter( 'postmeta_form_limit', 'meta_limit_increase' );
function meta_limit_increase( $limit ) {
    return 300; // change it accordingly
}

Ref: https://developer.wordpress.org/reference/hooks/postmeta_form_limit/