When you delete a NS resource record in the DNS Management Console the A record is NOT deleted, but the DNS pointer (PTR) resource record for the A record is properly deleted.

NOTE: The problem become apparent when you reopen the DNS Management Console, or when you use the shortcut menu to reload the zone.

This behavior occurs if the NS resource record points to a FQDN (Fully Qualified Domain Name) that is outside the zone.

To work around this issue, manually delete the host (A) record in the DNS Management Console.