Traffic Router currently requires the status of a given Traffic Router to be set to OFFLINE if it is to be removed from DNS answers. When set to OFFLINE, Traffic Ops will drop it from the CrConfig, so naturally it would drop from the configuration. If, however, a Traffic Router is set to ADMIN_DOWN, it remains in the CrConfig and Traffic Router leaves it in the DNS zones because the status is not OFFLINE.
Ideally, instead of checking if the status is OFFLINE, we would check to see if it's a "positive" status (ONLINE or REPORTED). Any other status should be considered invalid and the Traffic Router should be skipped with regard to DNS.