Sorry for the delayed response, many of us were at AstriCon last week.
When an endpoint registers for the first time, the endpoint is added to the contact but if an endpoint is just re-registering, the contact row isn’t updated with the endpoint name. So if some of your contacts existed before you upgraded to 13.11 and haven’t actually unregistered or expired since, the endpoint will be empty.
As a site note, if your situation allows it, you may want to consider letting the contact storage default to the internal astdb (based on sqlite3) rather than using a database. Since contacts are updated frequently, there’ll be a little less overhead and delay even with thousands of contacts. Subscriptions would fall into the same category.