Today, two posts were published using the custom achievement field.
Functions correctly here - http://www.empirestategamer.com/news/console/details-spilled-on-new-call-of-duty-black-ops-zombie-map-call-of-the-dead
Functions INCORRECTLY here - http://www.empirestategamer.com/news/console/dead-rising-2-sales-on-xbox-live-fantastic
As to why this is the case, I'm unaware.