When the Customer's domain's TLD is different than the Name Server's TLD, the DNS checkers show that the parent Name Servers are not sending out glue records for every Name Server listed - the server is sending out the Name Servers host names without sending the A records of those Name Servers. This happens if the Customers have Name Servers on different TLD's (for example domain.com with Name Server ns.domain.org - the .com root servers don't know the glue records for the .org servers). The Name servers' addresses are not provided by the parent (the TLD server) and had to be looked up individually.
Dear Customers, be assured that this is not a problem at all and the domains' configurations are okay.