Normally, all forward and reverse DNS records of a replica are removed when it is deleted. However, when a replica (say server.subdomain.example.com) is not in the primary domain (say example.com), its forward records are not removed when it is deleted. This is because !BindInstance.remove_master_dns_records always uses the primary domain when removing forward records (in my example, it would try to remove server.example.com instead of server.subdomain.example.com).